A composition for a heat cycle system and a heat cycle system employing the composition are provided. The composition has favorable lubricating properties and contains a working fluid for heat cycle and a refrigerant oil. The working fluid has a low global warming potential and can replace R410A. The working fluid contains an unsaturated fluorinated hydrocarbon compound having a specific structure. The refrigerant oil has a breakdown voltage of at least 25 kV, a hydroxyl value of at most 0.1 mgKOH/g, and a kinematic viscosity at 40° C. of from 5 to 200 mm2/s and a kinematic viscosity at 100° C. of from 2 to 30 mm2/s.