A working fluid for heat cycle, a composition for a heat cycle system containing the working fluid, and a heat cycle system employing the composition are provided. The working fluid has a low global warming potential and can replace R410A. The working fluid contains trifluoroethylene, 2,3,3,3-tetrafluoropropene, and optionally a hydrofluoroolefin other than trifluoroethylene and 2,3,3,3-tetrafluoropropene. The total proportion of trifluoroethylene and 2,3,3,3-tetrafluoropropene based on the entire amount of the working fluid is from 70 to 100 mass %.