I am using SharePoint online(Office 365). for office 365 it is necessary to create sandbox solution but in sandbox solution it is not possible to create visual web part. is there any way to create visual web part in sandbox solution ?
In sandbox solution you can not create visual webpart directly. for that you have download and install the Visual Studio 2010 SharePoint Power Tools from given link and then you can create Visual WebPart in sandbox solution. [Visual Web Part(Sandboxed)]
Another option to what Vikalp said is to use Visual Studio 2012
After you installed Visual Studio 2010 SharePoint Power Tools for you create a sandbox solutions, you need create a Empty Project, and after add a Visual Web Part (Sandboxed).
I tell this because only install VS 2010 Power Tools not enable this option.