To provide a composition for a heat cycle system having favorable lubricating properties and comprising a working fluid for heat cycle which has a low global warming potential and which can replace R410A, and a heat cycle system employing the composition.A composition for a heat cycle system, which comprises a working fluid for heat cycle containing 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 an aniline point of at least −100° C. and at most 0° C., and a heat cycle system employing the composition for a heat cycle system.