The role of Cl in halide hybrid perovskites CH3NH3PbI3(Cl) (MAPbI(3)(Cl)) on the augmentation of grain size is still unclear although many reports have referred to these phenomena. Herein, we synthesized MAPbI(3)(Cl) perovskite films by using excess MACl-containing precursors, which exhibited approximately an order of magnitude larger grain size with higher <110>-preferred orientation compared with that from stoichiometric precursors. Comprehensive mechanisms for the large grain evolution by Cl incorporation were elucidated in detail by correlating the changes in grain orientation, distribution of grain size, and the remaining Cl in the perovskite during thermal annealing. In the presence of Cl, <110>-and <001>-oriented grains grew faster than other grains at the initial stage of annealing. Further annealing led to the dissipation of Cl, resulting in the shrinkage of <001> grains while <110> grains continuously grew, as analyzed by x-ray rocking curve and diffraction. As a result of reduced grain boundaries and enhanced <110> texture, the trap density of perovskite solar cells diminished by similar to 10% by incorporating MACl in the precursor, resulting in a fill factor more than 80%.