use this css class
.rounded_corners
{
border: 1px solid Black;
-webkit-border-radius: 8px;
-moz-border-radius: 8px;
border-radius: 8px;
overflow: hidden;
}
.rounded_corners td, .rounded_corners th
{
border: 1px solid Black;
font-family: Arial;
font-size: 10pt;
text-align: center;
}
.rounded_corners table table td
{
border-style: none;
}
then set "rounded_corners" as your grid view css class, use following tag before
as
</Columns>
<HeaderStyle BackColor="#009900" ForeColor="Black" Height="30px" CssClass="gridheader"/>
<RowStyle BackColor="#FFD0AF" />
<AlternatingRowStyle BackColor="#BCF5A9" />
</asp:GridView>