A supercritical or transcritical Rankine cycle with ejector (ESRC, ETRC) based on the basic supercritical or transcritical Rankine cycle (SRC, TRC) are proposed for the conversion of low-grade heat to power in this paper. The thermodynamic comparative analyses on the SRC, TRC and ESRC, ETRC are conducted on the power output and thermal efficiency of the cycles. The carbon dioxide is chosen as the working fluid for the cycles. Water is chosen as the fluid of the low-grade heat. The water temperature is selected in a range of 60-90 degrees C, a typical water temperature is 80 degrees C, and the mass flow rate is I kg/s. The same temperature and mass flow rate of the water is the standard condition for the comparative analysis of the thermodynamic performance. Results show that the net power output of the cycles could be ranked from high to low: ESRC > ETRC > SRC > TRC, and the thermal efficiency could be ranked from high to low: TRC > SRC > ESRC > ETRC. (C) 2013 Elsevier Ltd. All rights reserved.