Using first-principles calculations, we investigate the electronic properties of the inorganic-organic hybrid perovskite CH3NH3PbI3, covering band structures, effective masses and exciton binding energies. For the first time, this study offers a comparison between perovskite polymorphs of CH3NH3PbI3 with GGA and HSE06 functionals. It is verified that the standard DFT-GGA is a reliable methodology for calculating the band gaps of Pb-based perovskites. Effective masses of cubic CH3NH3PbI3 are rather lighter than that of the orthorhombic and tetragonal polymorphs. And this characteristic echoes the lightest effective masses in the cubic CH3NH3PbI3. The reduced mass (0.05 m(0)) and binding energy (15.9 meV) are obtained for the cubic phase, providing direct evidence for the high efficiency of cubic polymorphs in solar energy applications. Besides, it is found that the phase transformation from orthorhombic to tetragonal phase leads to the reduction of the band gap.