By magnetization induced second harmonic generation (MSHG) the surface magnetization of fcc Fe films on Cu(100) is studied. The tetragonally distorted phase below 4 monolayers (ML) and the relaxed fcc phase above 4 ML can unambigously be distinguished by different average MSHG signals. Nearly constant asymmetry of the second harmonic intensities between 1.3 and 10 ML proves, to a good approximation, thickness independence of the surface magnetization irrespective of the phase transition at 4 ML. The nonlinear Kerr angle amounts to 4°.