This article describes a numerical study of the low emission window ( lower than 10 g/kg-f CO, 10 g/kg-f HC, 0.5 g/kg-f NO and almost soot-less) of diesel HCCI combustion. In order to locate low emission operating conditions, parametric calculations were performed over wide ranges by changing equivalence ratio, inlet temperature, intake valve closure timing, engine speed and fuel amount under the assumption of ideal homogeneous charge. The CO, HC, NO and soot emissions are summarized on equivalence ratio-peak cycle temperature maps, and the low emission region was found to be located in the region approximately from 1,600K to 1,800K peak cycle temperature, and on the lean side of stoichiometric equivalence ratio. In addition, it is revealed that clean HCCI combustion is possible with reduced EGR levels by retarding the intake valve closure timing, and the low emission window moves toward higher temperatures as engine speed is increased.