The development of liquefaction process for liquefied natural gas boil-off re-liquefaction plants will be addressed to provide an environmentally friendly and cost effective solution for gas transport. Onboard boil-off gas (BOG) re-liquefaction is a new technology that liquefies BOG and returns it to the cargo tanks instead of burning it. Exergetic efficiency optimization for cryogenic refrigeration cycle for re-liquefaction of LNG boil-off gas is performed. Thermodynamic modeling has been performed based on the energy and exergy analyses. Objective problem is developed based on maximization of the plant exergetic efficiency and selected decision variables and constraints. Optimization process is performed using MATLAB genetic algorithm optimization toolbox. The results of exergetic efficiency optimization are compared with the corresponding results of the base case system obtained in the previous study. Finally, effects of some operating parameters on the exergetic efficiency are discussed by sensitivity analysis.