In marine refrigeration there are currently used vapour compression refrigeration systems—technologies identified to be important energy consumers and traditional users of R134a refrigerant. It is important to tackle problems such as performance characteristics evaluation and adopting natural refrigerants instead of synthetics. Nano refrigerants are fluids with very good thermal properties, their use leading to the enhancement of vapour compression systems performance. In this paper it is analyzed the energy efficiency of such a system when a natural refrigerant (R600a) and a nano refrigerant (R600a/Al2O3) are alternatives to R134a. The thermodynamic analysis is established on the first and second laws of thermodynamics. The results show a performance improvement when using the nano refrigerant. Additional performance improvement it is reached when the refrigeration system is with sub cooling and super heating and when the volume concentration of Al2O3 nano particles increases.