At last I had fixed the issue by restyling the text char by char .See my solution
If rtfBox.SelectionFont.Bold Then
For x As Integer = selectionStart To selectionEnd - 1
rtfBox.Select(x, 1)
rtfBox.SelectionFont = VB6.FontChangeBold(rtfBox.SelectionFont, False)
Next
Else
For x As Integer = selectionStart To selectionEnd - 1
rtfBox.Select(x, 1)
rtfBox.SelectionFont = VB6.FontChangeBold(rtfBox.SelectionFont, True)
Next
End If