GraphicsPath path = new GraphicsPath();
            path.AddEllipse(this.pictureBox.ClientRectangle);
            this.pictureBox.Region = new Region(path);

------解决方案--------------------

用GraphicsPath 自己画一个六边形  
GraphicsPath gp = new GraphicsPath();
           gp.AddLine(74, 11, 144, 56);
           gp.AddLine(144, 56, 217, 5);
           gp.AddLine(217, 5, 233, 98);
           gp.AddLine(233, 98, 296, 129);
           gp.AddLine(296, 129, 224, 187);
           gp.AddLine(224, 187, 229, 255);
           gp.AddLine(229, 255, 144, 224);
           gp.AddLine(144, 224, 86, 260);
           gp.AddLine(86, 260, 67, 173);
           gp.AddLine(67, 173, 8, 138);
           gp.AddLine(8, 138, 78, 78);
           gp.CloseFigure();
           pictureBox1.Region = new Region(gp);
           return;