i just change foreach to for loop and its done...
If e.CommandName = "Del" Then
Dim dt As DataTable = TryCast(ViewState("add"), DataTable)
For i As Integer = 0 To dt.Rows.Count - 1
If (e.CommandArgument = dt.Rows(i)("cust_id").ToString) Then
dt.Rows.Remove(dt.Rows(i))
End If
Next
gvSavedetails.DataSource = dt
gvSavedetails.DataBind()
ViewState("add") = dt
End If
thnks to all commentates ...for helping..