If you have viewstate active for your control try on page_load event:
If not ispostback Then
Dim dt As New DataTable("Unit_tbl")
dt.Columns.Add("UnitName")
dt.Rows.Add("--Select UnitName--")
dt.Rows.Add("KOTAK")
dt.Rows.Add("ICICI BANK LTD")
UnitName_ddl.DataSource = dt
UnitName_ddl.DataBind
End If
If viewstate is disabled you have to load your values each time the page is postedback.