Additive engineering has been a crucial strategy to improve photovoltaic properties by reducing carrier trap centers or increasing carrier diffusion lengths in the lead halide perovskite active layer. We introduced a small amount of CH3NH3 +NO3 - (methylammonium nitrate, MAN) to a pristine MAPbI3 precursor solution as an agent for controlling the grain growth rate and healing iodine vacancies at grain boundaries of a perovskite film. With an addition of MAN, the larger grains were formed, confirmed by surface morphology images, and higher environmental stability was evidenced by in situ impedance spectroscopy results. For the optimized sample with an addition of 0.3 mol % MAN, high power conversion efficiency (PCE) of 20.5% (18.7% for the pristine sample) was successively demonstrated. In addition, the remnant NO3 ions were identified at the perovskite grain boundaries at room temperature and inside the unit cell at high temperature, as evidenced by results of in situ X-ray diffraction and in situ UV-visible spectroscopy measurements. Temperature dependence behaviors of the additive provide an in-depth understanding for designing and selecting additives to increase the PCE and stability of the perovskite solar cells.