The average direction of molecules and the absolute value of an effective second order nonlinear optical constant (\d (eff)\) were simultaneously determined using a new second harmonic generation (SHG) phase measurement with theoretical curve fitting. The theoretical curve fitting process was demonstrated using poled poly (methyl methacrylate) films doped with p-nitroaniline (PNA/PMMA), which were used as a reference and a sample. The second harmonic (SH) interference generated between the two poled PNA/PMMA films was modulated by rotating the phase generator inserted between the two. The fitting process derived d(eff) itself. The average direction of molecules was shown by the sign of d(eff). The obtained value of \d(eff)\ was verified by that from a Maker fringe measurement. The obtained average direction of molecules coincided with that determined by a poling geometry.