Hi you can this:
protected void RadGrid1_ItemDataBound(object sender, Telerik.Web.UI.GridItemEventArgs e)
{
if (e.Item is GridDataItem)
{
GridDataItem dataItem = (GridDataItem)e.Item;
dataItem["RowNumber"].Text = Convert.ToString(grdState.PageSize * grdState.CurrentPageIndex + e.Item.ItemIndex + 1);
}
}
<columns>
<telerik:gridboundcolumn uniquename="RowNumber" xmlns:telerik="#unknown"></telerik:gridboundcolumn></columns>