This paper reports on grain boundary (GB) roles in lead-free tin halide perovskite thin films. Nano scale spatial mapping of charge separation efficiency in methylammonium tin halide (MASn(I1-xBrx)(3), MA = CH3NH3) thin films were constructed by Kelvin probe force microscopy and conductive atomic force microscopy (C-AFM). We observed downward band bending at GBs under dark conditions and higher surface photovoltage along the GBs, confirmed by C-AFM which showed high local current flows along the GBs. The band bending degree and local current intensity were affected by the Br/I ratio. Photo-generated carriers were more effectively separated and collected at GBs with increased Br content, and hysteresis was observed in Br-rich Sn-halide perovskite.