A composition for a heat cycle system having favorable lubricating properties is provided. A heat cycle system employing the composition is also provided. The composition contains a working fluid for heat cycle, which has a low global warming potential and can replace R410A. The working fluid contains an unsaturated fluorinated hydrocarbon compound having a specific structure and a refrigerant oil having a breakdown voltage of at least 25 kV, a hydroxy value of at most 0.1 mgKOH/g, and a minimum temperature on the high temperature side of the phase separation temperature from the working fluid of at least 35° C. and a maximum temperature on the low temperature side of at most −60° C.