This paper describes a numerical study for analyzing combustion and emission characteristics and for Suggesting Optimal operating conditions of homogeneous charge compression ignition (HCCI) engines fueled with dimethyl ether (DME). Numerous calculations for DME HCCI engines were conducted for wide ranges of intake temperatures, equivalence ratios. and boost pressures. On the basis of the results for HCCI engines, engine performance and emissions including CO, HC, NOx, and soot are shown oil a peak cycle temperature and equivalence ratio map. In addition, combustion and emission characteristics of DME HCCI engines were compared to those of n-heptane HCCI engines for the same supplied lower heating values. Results of the present study showed that soot, HC, and NO, emissions are decreased but CO emission is increased by using DME instead of n-heptane for HCCI engines. Furthermore. the DME HCCI engine has a wider optimal operating range than the n-heptane HCCI engine.