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Nowadays, emission regulations and the requirement to reduce greenhouse gas emissions have escalated engine development efforts. In the present work, the effect of compression ratio on the performance, combustion and emission characteristics of a spark-ignition engine is evaluated at different operating conditions. A single-cylinder, water-cooled, spark-ignition engine (modified from a compression-ignition version) was used, with combustion chamber geometry consisted of flat cylinder head and a hemispherical bowl in the piston. Results showed that the brake thermal efficiency was increased from 9.8% to 12.9% when compression ratio was increased from 6.7:1 to 9.4:1 at low operating load of 5 N-m. Carbon monoxide emission was decreased when compression ratio was increased at all operating loads. However, as expected, nitric oxide emission was increased with the increase in compression ratio, with lower difference at low loads compared to medium and high loads.