A ternary solvent system consisting of dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL) and N-methyl-2-pyrrolidone (NMP) has been developed to improve the uniformity of CH3NH3PbI3 (MAPbI3) perovskite domains. Compared to MAPbI3 perovskite films prepared using a binary solvent consisting of DMSO and GBL, the surface roughness and uniformity of MAPbI3 films fabricated by using the ternary solvent system are greatly improved. The thermogravimetric analysis reveals that a NMP-PbI2-MAI intermediate, a DMSO-PbI2-MAI intermediate and MAPbI3 crystals co-existed in the as-cast MAPbI3 films. Furthermore, it is found that the thermal stability of intermediate phases and the solvent evaporation rate are critical for the nucleation of the perovskite crystals during the thermal annealing treatment. The thermally stable intermediates prepared with the ternary solvent are converted to MAPbI3 films with a highly uniform and smooth surface. The film forms an intimate contact with the charge transporting layer when the layer is applied as a photoactive layer in the solar cell. As a result, the power conversion efficiency of ternary solvent processed solar cells is enhanced by 38.2% compared to that of the binary solvent processed one. Furthermore, the stability of the ternary processed perovskite solar cells is greatly improved, as well. This investigation provides a better understanding about the role of different processing solvents or additives in effecting the perovskite film quality.