This article shows a research on the effect of in-cylinder flow on thermal stratified homogeneous charge compression ignition (HCCI) combustion to extend a high load limit of HCCI combustion. The study was conducted by three dimensional computational fluid dynamics coupled with simulation of chemical reactions. The in-cylinder temperature distribution was set as an initial condition. The in-cylinder flow was added before low temperature oxidation (LTO) and during high temperature oxidation (HTO), respectively. The results show that the in-cylinder flow before LTO leads to slow combustion due to heat loss to combustion chamber walls. The results also show that the in-cylinder flow during HTO is effective in fast combustion. This is mainly because in-cylinder flow expands hot burned mixture to unburned mixture, which accelerates the combustion.