In this study, we have discussed how grain sizes of CH3NH3PbI3 perovskites have an impact on the photovoltaic parameters such as a short-circuit current density Jsc, an open-circuit voltage Voc, and hence a power conversion efficiency PCE. As a result, we found that the device with larger grains exhibits the better photovoltaic parameters. The highest PCE was as high as 19.4%, which was obtained for the device with a grain size of ∼500 nm. We further discuss the efficiency limiting factors in terms of the grain size of CH3NH3PbI3 in perovskite solar cells.