try usnig the textrtf and selrtf properties
ie.

Private Sub Command1_Click ()
Open "mytext.rtf" For Output As 1
Print #1, RichTextBox1.SelRTF
Close 1
End Sub

or

Private Sub Command1_Click ()
Open "mytext.rtf" For Output As 1
Print #1, RichTextBox1.TextRTF
Close 1
End Sub