Form child =new Form();
point p = new point();
p.x = (pform.width-child.width)/2;
p.y = (pform.height-child.height)/2;
child.location = p;
child.toplevel =false;
child.bringtofornt();
child.startposition = formstartposition.manual;
child.show();
原文：http://www.cnblogs.com/zhangxiaoshuai/p/5212079.html