Based on molecular dynamics simulations, the plastic deformation of columnar polycrystalline silver nanowires under uniaxial tension at different temperatures has been studied systematically. At a temperature lower than 200 K, the system with large grains deforms predominantly via the mechanism of dislocation slip, and the maximum yield strength remains relatively constant with temperature. When the simulation temperature is higher than 200 K, grain sliding gradually becomes the dominant deformation mechanism, especially for those samples with reduced grain size and the maximum yield strength shows a decrease with the increase of temperature. The temperature-dependence of the Hall-Petch relation has been discussed.