How to Implement Nested DropDownLists Inside every rows in GridView?

To avoid server side postbacks, use jquery to get the current dropdown's selected item, and based on that, use ajax to bind the second dropdown.
As this dropdown will be within a grid and the id generated dynamically, we need to use ClientID to get the referred dropdown.

<templatecolumn>
<itemtemplate>
<asp:dropdown id="ddl_main" runat="server" ...="" onchange="javascript:ddl_MainUpdated('#' + '<%ddl_main.ClientID%>', '#' + '<%ddl_child.ClientID%>' );" xmlns:asp="#unknown" />   
<asp:dropdown id="ddl_child" runat="server" xmlns:asp="#unknown" />
</itemtemplate>
</templatecolumn>

javascript

function ddl_MainUpdated(mainDdlId, childDdlId) {
    var selectedItem = $(mainDdlId + ' option:selected').text();   // or val
    var listOfDependentItem = AjaxCall(selectedItem)  // this will be ajax method.... and will return an array of items.. <this is="" just="" set="" the="" items="" within="" ajax="" method="" itself="">
    listOfDependentItem.each( function (index) { childDdlId.append( $(this).value ) } );</this>

Please note that this is just an illustration of how code would work.