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 contains an unsaturated fluorinated hydrocarbon compound having a specific structure. The working fluid has a low global warming potential and can replace R410A. The refrigerant oil has a breakdown voltage of at least 25 kV, a hydroxyl value of at most 0.1 mgKOH/g, and an aniline point of at least −100° C. and at most 0° C.