Thank You M. Ogilvy for your answer but...

Private Sub Userform_Initialize()
For i = 1 To 6
ComboBox1.AddItem Worksheets("sheet1").Cells(1, i)
Next
End Sub

Your Example return only the value on my cells A1, B2, etc,
I would like appeared
Column A
Column B
Column C
in my ComboBox1
as the UserForm from "Data - Sort... - of Excel

Could I help me?

Thank you very much,

Michel