We present a new analytical method of molecular orientation in films using a phase measurement of second harmonic generation (SHG) with a theoretical curve fitting. The new analytical method provides the average direction of molecules and the absolute value of the effective second-order nonlinear optical constant (\d (eff)\) simultaneously. In conventional methods, the average direction of molecules in a sample has been relatively determined with that in a standard sample having preknown molecular direction. The second harmonic (SH) interference fringe obtained between a reference having preknown molecular direction and the sample has been compared with that between the reference and the standard sample. In the new method, however, the average direction of molecules can be obtained without the standard sample. The value of \d (eff)\ is calculated through the fitting process, the average direction of molecules is determined by the sign or the phase of d (eff) which is introduced by a reference. Complete expression with physical meanings is given for the valley points and the contrast of SH interference fringe. There, effect of a lens to focus a fundamental wave to a sample is also taken into account. The new phase measurement of SHG with the curve fitting has been carried out with poled poly(methyl methacrylate) films doped with 2 wt % of p-nitroaniline (PNA/PMMA), which were used as a reference and also as a sample. The accuracy of the new method has been demonstrated in comparison of the value \d (eff)\ of the sample with that determined by Maker fringe measurement. The determined average direction of molecules for the sample was confirmed by experimental geometry of the reference and the sample and also from the poling geometry for the sample. We have shown that the phase measurement of SHG and the curve fitting are useful tools to investigate the molecular orientation.