RadButton embedded in RadGrid is rendered with rounded corners only when ButtonType="StandardButton" (by default) and UseSubmitBehavior="true" (by default). In all other cases, it has sharp edges:
Code to replicate:
<telerik:RadButton ID="RadButton1" runat="server" Text="Outer button" RenderMode="Lightweight"></telerik:RadButton>
<telerik:RadGrid ID="RadGrid1" runat="server" AllowPaging="True" OnNeedDataSource="RadGrid1_NeedDataSource" RenderMode="Lightweight">
<MasterTableView AutoGenerateColumns="False">
<Columns>
<telerik:GridTemplateColumn HeaderText="TemplateColumn">
<ItemTemplate>
<telerik:RadButton ID="RadButton2" runat="server" Text="StandardSubmit" UseSubmitBehavior="true"></telerik:RadButton>
<telerik:RadButton ID="RadButton3" runat="server" Text="StandardNonSubmit" UseSubmitBehavior="false"></telerik:RadButton>
<telerik:RadButton ID="RadButton4" runat="server" Text="LinkButton" ButtonType="LinkButton"></telerik:RadButton>
<telerik:RadButton ID="RadButton5" runat="server" Text="SkinnedButton" ButtonType="SkinnedButton"></telerik:RadButton>
<telerik:RadButton ID="RadButton6" runat="server" Text="ToggleButton" ButtonType="ToggleButton"></telerik:RadButton>
</ItemTemplate>
</telerik:GridTemplateColumn>
</Columns>
</MasterTableView>
</telerik:RadGrid>
C#
protected void RadGrid1_NeedDataSource(object sender, GridNeedDataSourceEventArgs e)
{
(sender as RadGrid).DataSource = Enumerable.Range(1, 6);
}