Thank you Vahid but it is not working. i am trying to validate using javscript.

Attachment is requirde if dropdown value is 201. please advise.

here is my code:

<tr>
<td>
Type
</td>

<td>
<asp:DropDownList ID="ddlType" runat="server" Width="100%" CssClass="NormalSelect" onChange="change()">
</asp:DropDownList>
</td>
<td>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="Server" ErrorMessage=" Must enter Request Type" ControlToValidate="ddlType"
Display="dynamic" CssClass="ValidationError"></asp:RequiredFieldValidator>

</td>
</tr>
<tr>
<td >
Attachment
</td>
<td>
<asp:FileUpload ID="fileUpload" runat="server" CssClass="NormalSelect" style="display:none"></asp:FileUpload>
<asp:RequiredFieldValidator ID="uploadFileRequired" runat="Server" ControlToValidate="fileUpload" ClientValidationFunction="uploadFile_Validate"
Display="Dynamic" Text=" Upload file *" CssClass="ValidationError" ValidateEmptyText="true" />
</td>
</td>