در وب کانفیگ

<configuration>
  <appSettings/>
  <connectionStrings/>
  <system.web>
    <compilation debug="true" />

    <!-- Turn on Custom Errors -->
    <customErrors mode="On" 
      defaultRedirect="/404error.aspx">
      <error statusCode="404" redirect="/404Error.aspx"/>
    </customErrors>

  </system.web>
</configuration>