The objective of this work is to investigate the effects of water injection on performance of 5-stroke SI engine converted from the conventional 4-stroke SI engine. The cylinder head of the 2nd and 3rd cylinder was modified and connected to each other to be the ultra-expansion stroke or second expansion stroke (5th stroke). The engine was performed on a dynamometer at different engine speeds and loads of 1,500 to 3,000 rpm and 25% to 75% of full load, respectively. The water was injected during the second expansion stroke (5th stroke). Fuel and air consumptions, engine torque, rotational speed, air/fuel ratio and temperatures of intake air, engine oil, cooling water and exhaust gas were recorded to use for calculating brake specific fuel consumption (BSFC) and engine efficiency. The results showed that at low engine speed of 1,500 rpm for all loads, the water injection improves BSFC and the engine efficiency by 26% and 3.3%, respectively. However, a large amount of injected water shows insignificant improvement on BSFC and the engine efficiency due to short residence time, especially at high engine speed. The deterioration of engine oil can be observed after test. These findings are relevant to engine design or modification, especially at low engine speed.