Open sharepoint modal popup on page load

Hello All,

I have requirement to load Sharepoint modal popup and page load on the bases of some Querystring parameter values.

When I try to load scripts using "RegisterStartupScript", I get an error "SP.UI.$create_DialogOptions is not a function"

Later I add function ExecuteOrDelayUntilScriptLoaded(OpenPopupFn,'sp.js'); in the script but no effect.

still it is not loading sharepoint modal popup..

Every suggestions are appreciated.

Thanks,

Saumil Thakkar