The mechanism of axial stratification was investigated in a port injection SI engine. The port swirl and tumble number characterized the ports. The experiment consisted of a steady flow rig test for defining flow characteristics, engine test for finding lean misfire limit, fuel behavior tracing in a steady flow rig for the understanding basic stratification, fuel motion visualization in motored engine for the investigating real stratification process and flame capturing under known stratified conditions. The results show that not only swirl but also tumble affects axial stratification with the interaction of injection timing. It was also observed that stable initial flame improved engine stability by stratification, but the flame propagation pattern and direction was not affected by stratified condition, load, air-fuel ratio and fuel phase but by bulk in-cylinder flow pattern.