Organometallic halide perovskite solar cells such as MAPbI(3) have shown great promise as a low-cost, high-efficiency photovoltaic candidate. Recent studies demonstrated that by substituting an appropriate amount I ions of MAPbI(3) with Br ions, the device performance parameters, such as moisture stability or power conversion efficiency, can be further optimized. In this study, using time-resolved optical reflectivity to track the carrier dynamics in MAPb(I1-x,Br-x)(3) films with different Br contents, we found that photocarriers in MAPb(I1-x,Br-x)(3) films with x = 0.01 and 0.02 diffuse much faster than those in films with other Br contents. We suggest that the faster charge carrier diffusion benefits from larger crystal grain size. As a result, this suppresses electron and hole recombination and increases the carrier extraction efficiency, in agreement with the higher power conversion efficiency reported previously.