To provide a working fluid for heat cycle containing 1,2-difluoroethylene, which has high stability, and which has cycle performance sufficient as an alternative to R410A (a pseudoazeotropic mixture refrigerant of difluoromethane and pentafluoroethane in a mass ratio of 1:1) or 1,1,1,2-tetrafluoroethane while the influence over global warming is suppressed, a composition for a heat cycle system comprising it, and a heat cycle system employing the composition.A working fluid for heat cycle, which contains at least two members selected from a saturated hydrofluorocarbon and a hydrofluorocarbon having a carbon-carbon double bond other than 1,2-difluoroethylene, and 1,2-difluoroethylene.