The effects of 25 kinds of solute elements on hardness and grain size in annealed Pt-based binary alloys were investigated together with cold workability and a hardness variation with a cold rolling reduction. Gain size largely varied depending on solute elements and their concentrations, and so the decrement in grain size per 1 at% solute addition was evaluated based on grain size difference between pure platinum and a respective alloy. This value markedly reduced in the low solute concentration range below 5 at%, followed by a sluggish reduction in the higher solute concentration range. Hardness values obtained in all alloys were corrected considering contribution of hardening due to grain refinement, and solid solution hardening was evaluated by the increment in hardness per 1 at% solute addition. The lattice constants of pure platinum and alloys were measured to calculate the size misfits. The increment in hardness increased with the increase in the size misfit in all alloys, and this relationship was clearly divided into two groups depending on whether the binary alloy system is a completely miscible type or a type with the solubility limit. The increment in hardness in the latter type of alloys was much higher than that of the former, and this value became larger with the increase in the inverse value of the solubility limit in Pt-based binary alloy. The superior cold workability and very similar work hardening behavior were observed in all Pt-based binary alloys.