Actually it was SETpredefinedFrame.
And also the effect sometimes took effect sometimes don't.

I have trial and error around and by doing

m_DockManager.DocumentStyle.DocumentViewStyle = DocumentViewStyle.MdiStandard

change away from MdiStandard and then change it back again - will update the document frame.

However, doing so I lost position/size of mdistandard document windows.
So I am going to open different question.

I thought i have solved it but if i turn on skin e.g. macskin...
When change to another skin the mdistandard doc will still be in Macskin frame

Hello JSW W.,

The correct way to setup a predefined frame on a MdiStandard NUIdocument is the following:

NUIDocument doc = new NUIDocument();

NMdiChild host = doc.Host as NMdiChild;

if (host != null)

{

    NFrameAppearance frameAppearance = NUIManager.GetPredefinedFrame(PredefinedFrame.Simple);

    host.FrameAppearance = frameAppearance;

}

Regards,

Angel.