>> I was planning to migrate some of my project from FOX to .NET.
> But i was confuse if I use winform or using webform (ASP.nET). I always have a doubt that maybe soon porting it to WebForm would be the best since it can run at any platform.
>
> what do you think? should I port all may application web or using WebForm. are the any issue?
>
> Does winform has an advantage over Developement Cost(Time)?

Simplicio:

You don't give much details on what the app does or what you want to accomplish.

I don't see it as an advantage "over Developement Cost(Time)", as you say but mostly about flexibility.
What do want want out of the program?
Are users going to request browser access?
Do you prefer to install an EXE on the client (a fat app like in VFP)?

Winforms have a richer/more powerful interface. But it must be installed in the client (program plus .Net runtime).
Webforms, just give you the power of the Internet/(Intranet). Nothing to deliver or install. All they need is a browser (like Internet Explorer or Firefox) and maybe procer access credentials (login/password).

If your application has an N-Tier design, with business objects taking care of the business rules and data access, then you can provide both Winforms and Webforms as needed. If your logic is in a Winform then you can't easily separate it later.

Both formats have pluses and minuses. It is ultimately your decision (or the client's) which way you want to go.