To reduce heat transfer between hot gas and cavity wall, thin Zirconia (ZrO2) layer (0.5mm) on the cavity surface of a forged steel piston was firstly formed by thermal spray coating aiming higher surface temperature swing precisely synchronized with flame temperature near the wall resulting in the reduction of temperature difference. However, no apparent difference in the heat loss was analyzed. To find out the reason why the heat loss was not so improved, direct observation of flame impingement to the cavity wall was carried out with the top view visualization technique, for which one of the exhaust valves was modified to a sapphire window. Local flame behavior very close to the wall was compared by macrophotography. Numerical analysis by utilizing a three-dimensional simulation was also carried out to investigate the effect of several parameters on the heat transfer coefficient. From the observation of wall impinged flame, it was revealed that a kind of thermal boundary layer with Zirconia coating was thinner than the baseline, which could be resulted in the increase in heat transfer coefficient. Furthermore, the numerical simulation results suggested that higher wall surface temperature swing with Zirconia coating is not the main cause of thinner boundary layer, but surface roughness and/or porous structure is. To confirm the hypothesis, new pistons with different insulating structures were then experimented. Even though the heat loss was not so improved because of the limited area of insulation, the potential for BTE improvement was confirmed.