Hi Karsten,
thanks very much for your help. yes, you understood me well, and I checked the link you provided.
But in my case, I use a BrowserFormWebPart in a SharePoint page to show the infopath form, not the xsn or xml, so I still do not know how to populate date to the form
at load time.
The BrowserFormWebPart is under the namespace of Microsoft.Office.InfoPath.Server.Controls.WebUI , and SharePoint uses this web part to dispaly/edit infopath form by default.
Thanks