Metal silver in the nanoscale range is widely used in the fields, such as microelectronics, optoelectronics, catalysis etc., due to its unique electrical and thermal conductivity, especially in nano-electrodes and nano-devices. Tensile deformation behavior of polycrystalline silver nanowires, in various grain size, is simulated using molecular dynamics method. Effect of grain size on elastic modulus, yield strength, and plastic deformation mechanism of polycrystalline silver nanowire is analyzed in detail. Results indicate that polycrystalline silver nanowires show a softening for grain sizes smaller than 13.49 nm, a reverse Hall-Petch relationship. At this stage, the plastic deformation is dominated by ‘sliding’ at the grain boundaries and rotating of grains; moreover, a five-fold twin is formed at the later stage of deformation. While the plastic deformation mechanism changes to dislocation sliding, when the grain size is larger than 13.49 nm, and a large number of twins are formed at the later stage of deformation.