The amount of improvement of the cooling output from the subcooling power has not been adequately studied by thermodynamics, and the corresponding trend has still not been realized. Therefore, this paper contributes to the understanding of the thermodynamic relationship of the subcooling power and increase of the cooling capacity. The parameter RICOSP, defined as the ratio of the rise in the cooling output to the subcooling power, is employed as the critical indicator and its expression is derived. A vapour compression chiller with subcooling is simulated to analyse RICOSP under different working conditions. It is shown that the subcooling power cannot be fully transformed into an increase of the cooling output as the speed of compressor, inlet temperature and flow rate of cooling water as well as chilled water for the chiller with the subcooling is identical to that for the system without the subcooling. Additionally, the dependence of RICOSP on the speed of the compressor, as well as on the flow rate of cooling water and chilled water, is strong. RICOSP increases by 17.4% and 8% when the speed of compressor and flow rate of cooling water are reduced by 80%, respectively. RICOSP decreases by 22.5% when the flow rate of chilled water is reduced by 80%. However, RICOSP is nearly independent of the inlet temperature of cooling water and chilled water. This paper deepens the understanding of a vapour compression chiller with subcooling. (C) 2017 Elsevier Ltd. All rights reserved.