Private Sub ConstructFontWithString(ByVal e As PaintEventArgs)
Dim font1 As New Font("Arial", 20)
e.Graphics.DrawString("Arial Font", font1, Brushes.Red, New PointF(10, 10))
End Sub
↑这是MSDN给出的例子
font类字体需要由构造器指定，除了Font("xxx", xxx)以外还有别的重载
参见
http://technet.microsoft.com/zh-cn/library/system.drawing.font.aspx

RichTextBox1.Font = New System.Drawing.Font("SimKai", 25)

richtextbox1.Font = New Font("楷体", 26, FontStyle.Bold)
richtextbox1..ForeColor =Color.Blue