In this study, the effects of steam injection at different injection rates on the evaluations of performance parameters and emissions of a gasoline engine have been investigated. Electronically controlled steam injection method has been used to inject the steam into the engine. The optimum steam ratio has been determined as 20% of fuel mass (S20) in terms of performance and emission parameters. Steam injected gasoline engine has been modeled by using zero-dimensional two-zone combustion model for optimum steam ratio at full load condition. The obtained results have been compared with conventional gasoline engine in terms of performance and NO, CO, CO2, HC emissions. The results of theoretical combustion model agree with experimental data quite well. In the experimental results, it is seen that the engine torque and the effective power increase up to 4.65% at 3200 rpm, specific fuel consumption reduces up to 6.44% at 2000 rpm. There is 40% average reduction in NO emissions at 2800 rpm and it is 31.5% in HC emissions at 2000 rpm. (C) 2013 Elsevier Ltd. All rights reserved.