To find more effective lean mixture preparation methods for smokeless and low NOx combustion, a numerical study of the effects of in-cylinder flow field before injection on mixture formation in a premixed compression ignition engine was conducted. Premixed compression ignition combustion is a very attractive method to reduce both NOx and soot emissions, but it still has some problems, such as high HC and CO emissions. In case of early direct injection, it is important to avoid wall wetting by spray impingement, which can cause higher HC and CO emissions. Since it is not easy to examine the effects of initial flow and injection parameters on mixture formation over the wide range by practical engine tests, a computer program named “GTT (Generalized Tank and Tube)” code was used to simulate the in-cylinder phenomena before autoignition. Different combinations of the initial flow (including tumble flow) and fuel injection parameters were tested to predict the effects of the flow field on the fuel-air mixture formation. Based on the calculation results, possibilities to form leaner mixtures, avoiding wall wetting before autoignition, are discussed. The calculation results indicated that the in-cylinder flow field before fuel injection should be adapted to fuel injection conditions, so that fuel droplets are dispersed more widely before complete evaporation to make a leaner mixture.