Richtextbox problem about Appending text

Hi, the resulting text supposed to be
<b>Highlighted</b> No Highlighted
(<b>..</b> is the portion that bold)
But the result appeared to be all bold, How to solve it?

Followed is the code: (rtf is the name of richtextbox)

Private Sub Command1_Click()
    Dim index As Integer
    Dim iStart As Integer
    Me.rtf.Text = ""
    Dim iLength As Integer
    iStart = Len(rtf.Text)
    rtfAppend rtf, "Highlighted "
    iLength = Len(rtf.Text) - iStart
    rtf.SelStart = iStart
    rtf.SelLength = iLength
    rtf.SelBold = True
    rtfAppend rtf, " No Highlight"
End Sub


Private Sub rtfAppend(ByVal rtf As RichTextBox, ByVal str As String)
    rtf.SelStart = Len(rtf.Text)
    rtf.SelLength = 0
    rtf.SelText = str
    'rtfComponent.SelBold = False
End Sub