thanks everybody for your valued comments...i have solved my problem by doing following.
Private Sub btnBold_Click(sender As Object, e As EventArgs)
If btnBold.BackColor = Color.White Then
For Each c As Control In Me.pnlDesign.Controls
If TypeOf c Is TextBox AndAlso c.Name.ToString = "txt" & tNo Then
c.Font = New Font(cboFont.Text, Convert.ToSingle(cboSize.Text), FontStyle.bold)
End If
Next
btnBold.BackColor = Color.Aqua
Else
For Each c As Control In Me.pnlDesign.Controls
If TypeOf c Is TextBox AndAlso c.Name.ToString = "txt" & tNo Then
c.Font = New Font(cboFont.Text, Convert.ToSingle(cboSize.Text), FontStyle.Regular)
End If
Next
btnBold.BackColor = Color.White
End If
End Sub