But what if the combo box is in a user form?! Like this user form will pop up
everytime the user run the macro.

"merjet" wrote:

You don't say how the ComboBox is populated initially. But suppose
ComboBox1 is initially populated from a list in Sheet1!A1:An. If the
user adds a new item to ComboBox1, the following code will add the new
item to the list. DO NOT use RowSource in the Properties Window.

Private Sub UserForm_Activate()
Dim iEnd As Long
Dim rng As Range
iEnd = Sheets("Sheet1").Range("A1").End(xlDown).Row
Set rng = Sheets("Sheet1").Range("A1:A" & iEnd)
ComboBox1.RowSource = rng.Worksheet.Name & "!" & rng.Address
End Sub

Private Sub ComboBox1_Change()
Dim rng As Range
Set rng = Range(ComboBox1.RowSource)
If IsError(Application.Match(ComboBox1.Value, rng, 0)) = True _
And ComboBox1.Value < "" Then
Set rng = rng.Resize(rng.Rows.Count + 1, 1)
rng(rng.Rows.Count, 1) = ComboBox1.Value
ComboBox1.RowSource = rng.Worksheet.Name & "!" & rng.Address
End If
End Sub

Hth,
Merjet