Research has been conducted on Controlled Auto-Ignition (CAI) engine with natural gas. CAI engine has the potential to be highly efficient and to produce low emissions. CAI engine is potentially applicable to automobile engine. However due to narrow operating range, CAI engine for automobile engine which require various speed and load in real world operation is still remaining at research level. In comparison some natural gas engines for electricity generation only require continuous operation at constant load. There is possibility of efficiency enhancement by CAI combustion which is running same speed at constant load.Since natural gas is primary consisting of methane (CH4), high auto-ignition temperature is required to occur stable auto-ignition. Usually additional intake heat required to keep stable auto-ignition. To keep high compression temperature, single cylinder natural gas engine with high compression ratio (CR=26) was constructed.Since high compression ratio (CR=26), CAI engine with natural gas would improve thermal efficiency due to higher theoretical thermal efficiency. However, indicated thermal efficiency of CAI engine does not improved as expected due to increasing heat loss from cylinder wall. To increase indicated thermal efficiency, combustion chamber shape modification is also required.To analyze combustion chamber shape effect, comparison between standard engine with same stroke and bore (S/B=1.0) and modified engine with same displacement, longer stroke and smaller bore (S/B=2.1) was conducted. Brake thermal efficiency of longer stroke engine (S/B=2.1) was 39.1% while keeping low NOx level, which was an increase of 9.2 points over the 29.9% for the conventional engine (S/B=1.0). Heat release analysis shows major factor of enhanced efficiency is reduced heat loss from cylinder wall and exhausted unburned fuel.