To provide a composition for a heat cycle system with which a HCFO or a CFO can more stably lubricate, and a heat cycle system employing the composition. A composition for a heat cycle system comprising a working fluid for heat cycle containing at least one compound selected from predetermined HCFO and CFO, and a mixed refrigeration oil obtained by mixing a naphthenic mineral oil and other predetermined refrigeration oil, wherein the mixed refrigeration oil has a kinematic viscosity at 40° C. of 300 mm2/s or lower, a mixed composition 1 of the working fluid for heat cycle and the mixed refrigeration oil at a concentration of the working fluid for heat cycle of 10 mass % has a viscosity at 60° C. of 10 mPas or higher, and a mixed composition 2 of the working fluid for a heat cycle system and the mixed refrigeration oil at a concentration of the mixed refrigeration oil of 5 mass % has a two phase separation temperature of 0° C. or lower, and a heat cycle system, which employs the composition for a heat cycle system.