showdialog() methos not working

Question

hi everyone,
i created  two forms, one is  parentForm and another one is childForm.
i place one button on parentForm(btnClick).In button Click event i called this below function
button click event
 thread obj = new thread(LoadChild);
obj.backgroundprocess = true;
obj.start();
public void LoadChild()
{
chidForm objChild = new childForm();
objChild.ShowDialog();
}
i set parentForm property isMDiContent is true..
and my problem is whenever i am calling this function
childForm openning fine but parentForm also openning without closing childForm..
plz give me anyone solutin...