Dim pCell As HMergedCell
 Dim nOffset As Integer
 Dim i As Integer, j As Integer
 
Private Sub DataGridView1_CellEndEdit(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.DataGridViewCellEventArgs) Handles _
  DataGridView1.CellEndEdit
....
                    For k As Integer = nOffset To nOffset + 3
                        Me.DataGridView1.Rows(i).Cells(k) = New HMergedCell()
                        pCell = DirectCast(Me.DataGridView1.Rows(i).Cells(k), _
                          HMergedCell)
                        pCell.LeftColumn = nOffset
                        pCell.RightColumn = nOffset + 3
                    Next
                    nOffset += 7
                  ...
    End Sub

 
    Public Class HMergedCell
        Inherits DataGridViewTextBoxCell
        Private m_nLeftColumn As Integer = 0
        Private m_nRightColumn As Integer = 0
 
        Public Property LeftColumn() As Integer
            Get
                Return m_nLeftColumn
            End Get
            Set(ByVal value As Integer)
                m_nLeftColumn = value
            End Set
        End Property
 
        Public Property RightColumn() As Integer
            Get
                Return m_nRightColumn
            End Get
            Set(ByVal value As Integer)
                m_nRightColumn = value
            End Set
        End Property
 
        Protected Overrides Sub Paint(ByVal graphics As Graphics, ByVal _
          clipBounds As Rectangle, ByVal cellBounds As Rectangle, ByVal _
          rowIndex As Integer, ByVal cellState As DataGridViewElementStates, _
          ByVal value As Object, _
         ByVal formattedValue As Object, ByVal errorText As String, ByVal _
         cellStyle As DataGridViewCellStyle, ByVal advancedBorderStyle As _
         DataGridViewAdvancedBorderStyle, ByVal paintParts As _
         DataGridViewPaintParts)
                Try
            Dim mergeindex As Integer = ColumnIndex - m_nLeftColumn
            Dim i As Integer
            Dim nWidth As Integer
            Dim nWidthLeft As Integer
            Dim strText As String
 
            Dim pen As New Pen(Brushes.Black)
 
            ' Draw the background
            graphics.FillRectangle(New SolidBrush(System.Drawing.Color.White), _
              cellBounds)
 
            ' Draw the separator for rows
            graphics.DrawLine(New Pen(New SolidBrush( _
              SystemColors.ControlDark)), cellBounds.Left, cellBounds.Bottom - _
              1, cellBounds.Right, cellBounds.Bottom - 1)
 
            ' Draw the right vertical line for the cell
            If ColumnIndex = m_nRightColumn Then
                graphics.DrawLine(New Pen(New SolidBrush( _
                  SystemColors.ControlDark)), cellBounds.Right - 1, _
                  cellBounds.Top, cellBounds.Right - 1, cellBounds.Bottom)
            End If
 
            ' Draw the text
            Dim rectDest As RectangleF = RectangleF.Empty
            Dim sf As New StringFormat()
            sf.Alignment = StringAlignment.Near
            sf.LineAlignment = StringAlignment.Near
            sf.Trimming = StringTrimming.EllipsisCharacter
 
            ' Determine the total width of the merged cell
            nWidth = 0
            For i = m_nLeftColumn To m_nRightColumn
                nWidth += Me.OwningRow.Cells(i).Size.Width
            Next
 
            ' Determine the width before the current cell.
            nWidthLeft = 0
            For i = m_nLeftColumn To ColumnIndex - 1
                nWidthLeft += Me.OwningRow.Cells(i).Size.Width
            Next
 
            ' Retrieve the text to be displayed
            strText = Me.OwningRow.Cells(m_nLeftColumn).Value.ToString()
 
            rectDest = New RectangleF(cellBounds.Left - nWidthLeft, _
              cellBounds.Top, nWidth, cellBounds.Height)
            graphics.DrawString(strText, New Font("Arial", 10, _
            FontStyle.Regular), Brushes.Black, rectDest, sf)
        Catch ex As Exception
            Trace.WriteLine(ex.ToString())
        End Try
        End Sub
 
    End Class

Danke für die Hilfe

Gruß Christian