The consumption of powerful greenhouse gases should be drastically reduced worldwide due to the serious problem of global warming. Natural refrigerants CO2 is the most promising refrigerants and has been widely applied in small refrigeration systems because of the low global warming potential (GWP) and no ozone depletion potential (ODP). It is universally acknowledged that an optimum refrigerant charge existing in a small refrigerant system can maximize the coefficient of performance (COP) of system. In the present work, the void fraction method with 6 classic models was used to predict the optimum refrigerant charge of a small CO2 water-source heat pump water heater. Based on an existing unit in the lab, the changes of system performance were studied with various refrigerant charges by a series of relevant experiments and the optimum charge was obtained to verify the accuracy of all the void fraction models in the references. The results indicated that each model had a relatively good accuracy in determining the optimum refrigerant charge of the tested unit with the error less than 9.80%. Hughmark model could be recommended to predict the optimum refrigerant charge of the studied system because of the negligible errors by less than 2.59%. Moreover, in the view of heating energy efficiency ratio (EERheat), the value calculated by the Hughmark model was almost the same with that obtained from the experimental measurement. On the basis of a small system with a transcritical cycle, the optimum charge calculated by void fraction method was more accurate than the ones from experimental data method and rated working condition method. The present work can provide the guidelines of predicting the optimum refrigerant charge in small CO2 systems.