Windows Forms DataGridView 没有提供合并单元格的功能，要实现合并单元格的功能就要在CellPainting事件中使用Graphics.DrawLine和 Graphics.DrawString 自己来“画”。

下面的代码可以对DataGridView第1列内容相同的单元格进行合并：
        private void dataGridView1_CellPainting(object sender, DataGridViewCellPaintingEventArgs e)
        {
            // 对第1列相同单元格进行合并
            if (e.ColumnIndex == 0 && e.RowIndex != -1)
            {
                using
                    (
                    Brush gridBrush = new SolidBrush(this.dataGridView1.GridColor),
                    backColorBrush = new SolidBrush(e.CellStyle.BackColor)
                    )
                {
                    using (Pen gridLinePen = new Pen(gridBrush))
                    {
                        // 清除单元格
                        e.Graphics.FillRectangle(backColorBrush, e.CellBounds);

// 画 Grid 边线（仅画单元格的底边线和右边线）
                        //   如果下一行和当前行的数据不同，则在当前的单元格画一条底边线
                        if (e.RowIndex < dataGridView1.Rows.Count - 1 &&
                        dataGridView1.Rows[e.RowIndex + 1].Cells[e.ColumnIndex].Value.ToString() != 
                        e.Value.ToString())
                            e.Graphics.DrawLine(gridLinePen, e.CellBounds.Left,
                            e.CellBounds.Bottom - 1, e.CellBounds.Right - 1,
                            e.CellBounds.Bottom - 1);
                        // 画右边线
                        e.Graphics.DrawLine(gridLinePen, e.CellBounds.Right - 1,
                            e.CellBounds.Top, e.CellBounds.Right - 1,
                            e.CellBounds.Bottom);

// 画（填写）单元格内容，相同的内容的单元格只填写第一个
                        if (e.Value != null)
                        {
                            if (e.RowIndex > 0 &&
                            dataGridView1.Rows[e.RowIndex - 1].Cells[e.ColumnIndex].Value.ToString() == 
                            e.Value.ToString())
                            { }
                            else
                            {
                                e.Graphics.DrawString((String)e.Value, e.CellStyle.Font,
                                    Brushes.Black, e.CellBounds.X + 2,
                                    e.CellBounds.Y + 5, StringFormat.GenericDefault);
                            }
                        }
                        //e.Handled=true;这一句非常重要，必须加上，要不所画的内容就被后面的Painting事件刷新不见了！！！
                        e.Handled = true;
                    }
                }
            }