Since China accelerated its market oriented economic reforms at the end of 1992, its energy intensity has declined 3.6% annually over 1993-2005. However, its energy intensity declined 4.2% annually during its first reform period 1979-1992. Therefore, can we conclude that the accelerated marketization since the end of 1992 has made no contribution to its energy efficiency improvement? In order to answer this challenging question, we examine the changes of energy own-price elasticity, as well as the elasticities of substitution between energy and non-energy (capital and labor) in China during the periods of 1979-1992 and 1993-2003. Generally, in transition or developing economies, holding the technology and output level fixed, if the energy own-price elasticity (algebraic value) declines or the substitution elasticity between factors rises, they will contribute to energy efficiency improvement. Our empirical study finds that: (1) during 1979-1992, the energy own-price elasticity is positive (0.285), and capital-energy, labor-energy are both Morishima complementary; which indicates a distorted energy price and inefficient allocation; and (2) during 1993-2003, the own-price elasticity for energy is negative (-1.236), and capital-energy and labor-energy are both Morishima substitute. All factor demands become more elastic, and all elasticities of substitution increase. The implication is that the accelerated marketization contributes substantially to energy efficiency improvement since 1993. (c) 2006 Elsevier Ltd. All rights reserved.