A rationally based algorithm was proposed to evaluate the optimal mass charge into refrigerating machines. The calculated results indicate that the system performance is strongly related to the refrigerant mass charge. In the early stage of the refrigerant charge process, a sharp rise of COP is observed. The COP reaches an optimal value for a specified refrigerant charge and shows a slight drop for a further increase of refrigerant charge. Calculated results reveal similar trends to those of experimental data. The analysis of the study can well interpret the cycle transformation subject to refrigerant mass charge.