The purpose of incorporating an ejector into vapour compression cycle is to improve the COP by reducing the throttling loss associated with the expansion device. A computer simulation of the improved cycle is carried out using a one-dimensional model based on mass, momentum and energy balances. Refrigerant characteristics were evaluated using NIST subroutines for equations of state solutions. According to the results of simulation of the improved cycle, it has been shown that the geometric parameters of the ejector design have considerable effects on the system's performance. The maximum COP is obtained for Phi(opt) whose value is around 10. Several refrigerants are considered; it has been observed, at Phi(opt) and for given operating conditions, that the best performances are obtained with R141b. Compared with the standard cycle the COP of the improved cycle shows an increase of about 22%. Copyright (c) 2006 John Wiley & Sons, Ltd.