Optical properties of perovskite CH3NH3PbI3 are investigated in three cubic, tetragonal and orthorhombic phases.  Using density functional theory and with GGA approximation, the real and imaginary parts of dielectric function, refractive index and absorption coefficient are computed. The results show that optically the perovskite is an anisotropic material in two orthorhombic and tetragonal phases that show birefringence property. Cubic phase has a better absorption coefficient than the two other phases in visible region