A lubricating oil for a refrigeration system using a tetrafluoroethane refrigerant containing a fluorinated compound represented by formula (I), wherein X represents a monovalent group having a multiple bond, A represents a mono-, di- or trivalent residue of an unsubstituted or partially substituted perfluorocarbon, perfluoro ether or perfluoro polyether. This oil has not only a good compatibility with a tetrafluoroethane refrigerant represented by HFC-134a in a wide temperature range but also excellent properties including heat resistance, lubrication behaviors, electric insulation, and viscosity-to-temperature relationship.