In polyol synthesis at relatively low concentrations of polyvinylpyrrolidone (PVP, surface-capping agent), some micrometer-size Ag truncated decahedra with five-fold-twinned structure were clearly observed. This result indicates that the internal strain in the five-fold-twinned crystals is not sufficient to restrict their lateral growth. On the basis of first-principles calculation of their surface energies before and after the adsorption of surface-capping agent, it is proposed that the surface energy difference between the (100) and (111) facets (Delta phi(100)-(111)) plays an essential role in the aggressive growth of lateral (100) facets, overcoming the strain restriction. When enough PVP is adsorbed on the surfaces of Ag crystals, the Delta phi(100)-(111) will evidently decrease, and then the strain restriction on the lateral growth becomes predominant, resulting in pentagonal anisotropic structures. Following this mechanism, diameter-tunable Ag wires have been synthesized by controlling the axial growth and the lateral growth separately at higher and lower PVP concentrations, respectively.