Try this
aspx
<asp:Repeater ID="rptrPackage" runat="server">
<ItemTemplate>
<asp:Label ID="Pack" runat="server" Text='<%# Bind("Pack") %>'></asp:Label>
<asp:HiddenField ID="hID" Value='<%# Eval("PackageID") %>' runat="server" />
<asp:Repeater runat="server" ID="rptDetails">
<HeaderTemplate>
<ul>
</HeaderTemplate>
<ItemTemplate>
<li>
<%# Eval("Details")%>
</li>
</ItemTemplate>
<FooterTemplate>
</ul>
</FooterTemplate>
</asp:Repeater>
</ItemTemplate>
</asp:Repeater>
code behind
private void Populate()
{
DataTable dtPackage = tblPackage();
DataTable dtDetails = tblDetails();
rptrPackage.DataSource = dtPackage;
rptrPackage.DataBind();
foreach (RepeaterItem item in rptrPackage.Items)
{
Repeater rptDetails = ((Repeater)item.FindControl("rptDetails"));
rptDetails.DataSource = dtDetails.Select("PackageID = " + ((HiddenField)item.FindControl("hID")).Value).CopyToDataTable();
rptDetails.DataBind();
}
}