OK I found the solution !!
I used this code :
shape2 = slides(slideNo).Shapes.AddTable(500, 500, 5000, 800, 4, 5)
shape2.MergeCells(shape2.GetCell(3, 4), shape2.GetCell(4, 4))
shape2.MergeCells(shape2.GetCell(5, 4), shape2.GetCell(5, 4))
When i merge the cells of first row there is no problem , but when i try to merge cells in below rows. It is not necessary that all cells of row be merged.
The result i get is as in attached image
Dear Sanjeev,
Thank you, we will fix it in the next version.
Dear Sanjeev,
Please check 2.1.2 hot fix.