Regarding the polycrystalline nature of the solution-processed organic/inorganic hybrid perovskite (ABX3) thin films, how to enhance the crystallinity of the prepared perovskite films has become the most critical issue for realizing high-performance perovskite solar cells. Recently, A-site cation engineering has been reported to effectively modulate the perovskite crystallization. Nevertheless, owing to the limited tolerance space of three-dimensional (3D) perovskite, the suitable A-site cations are extremely restrained to methylammonium (MA), formamidinium (FA), or Cesium (Cs) cations. We herein described a new A-site cation engineering for CH3NH3PbI3 by utilizing imidazolium (IA) cation that affords a tolerance factor of 1 to modulate the perovskite crystallization. The mixed MA0.95IA0.05PbI3 film was revealed to possess an improved film quality and crystallinity as compared to the pristine MAPbI3 film, which thereby enables a much increased carrier lifetime as a result of the reduced defect density. Consequently, its derived solar cell device yields both enhanced efficiency (17%) and stability, outperforming the control MAPbI3 device showing an inferior efficiency of 15.7%.