Rad Form TitleBar Collapse

Hello Donghee,

According to the attached picture, it seems that you have an MDI RadForm and you want to remove the title bar of the child form. In order to achieve this, you should set the FormBorderStyle property of the child form to None. 

RadForm childform = new RadForm();
this.IsMdiContainer = true;
childform.MdiParent = this;
childform.FormBorderStyle = FormBorderStyle.None;
childform.Show();

The FormBorderStyle property mimics the behavior of the same property of a standard windows form. A similar approach is demonstrated here:
https://stackoverflow.com/questions/50076948/remove-top-bar-on-child-form

I hope this helps. Please let me know if there is anything else I can help you with. 

Regards,
Nadya
Progress Telerik