One of the major problems associated with HCCI combustion engine application is lack of direct control for combustion timing. A proposed solution for combustion timing control is using a binary fuel blend in which two fuels with different auto-ignition characteristics are blended at various ratios on a cycle-by-cycle basis. The aim of this research is to investigate the exergy analysis of HCCI combustion when a blended fuel, which consists of n-heptane and natural gas, is used. In order to accomplish this task, a single-zone combustion model has been developed, which performs combustion computations using a complete chemical kinetics mechanism. The study was carried out with different percentages of natural gas in blended fuels and EGR (exhaust gas recirculation) ranging from about 45 to 85 percent and 0 to 40 percent, respectively. The results reveal that, when mass percentage of natural gas increases, exergy destruction is decreased increasing the second-law efficiency. Introducing EGR into the intake charge of dual fuel HCCI engine up to some stage (optimum value) enhances the second-law performance of the engine in spite of a reduction in work. (C) 2011 Elsevier Ltd. All rights reserved.