In order to determine the optimum charge of a display cabinet CO

2

refrigeration system and study the effect of the refrigerant charge on the system performance

the experimental data method and the rated operating method were used to predict the refrigerant charge of the system

and then the experiments were conducted at different charges. The results show that the calculation methods for small conventional refrigeration systems can also be applied to the CO

2

system with a very small calculation error. The calculation results of the experimental data method are almost the same as those of the rated operating method for the small CO

2

refrigeration system. For the CO

2

transcritical system

the pseudo critical state occurs after the sy

stem works for a while when the system adopts regenerative cycle

and the suction temperature and discharge temperature drop suddenly. The results of this study may be useful for determining optimal refrigerant charge and maintaining efficient operation of small CO

2

refrigeration systems.