The polymer of N-phthalimidomethyl methacrylate (PNPMMA) was synthesized by free radical polymerization. The poly(PNPMMA) is significant material, because it is a soluble material. The poly(PNPMMA) exhibits a low transmittance behavior in the visible region. The band gap of the poly(PNPMMA) thin film was found to be 1.62 eV. The refractive index value of the poly(PNPMMA) varies between 3.00 and 1.56. The plot of refractive index is composed of two regions so-called normal and abnormal dispersion. We calculated many optical parameters such as average transmittance, absorption band edge, refractive index, incidence and refraction angle, direct and indirect band gap, real and imaginary parts of dielectric constant, real part and imaginary parts of the optical conductivity of the poly(PNPMMA). The poly(PNPMMA) is a semiconductor and it can be used in the fabrication of the metal-semiconductor (MS) contacts like Schottky diode due to low optical band gap and suitable optical contacts and it is significant material in applications of some devices such as an image sensor packaging structure and high intensity medium-pressure lamps due to low transmittance values in the visible region.