The effect of atmospheric pressure on the performance characteristics of a crankcase-scavenged spark-ignition two-stroke cycle engine has been investigated. An air-borne, 350 cc opposed 2-piston engine was tested on an experimental test bench where both the pressure at the inlet manifold and the pressure at the exhaust pipe were controlled separately.The experimental results were analyzed by using a computer program, the MICE Program, which simulates in detail the various processes occurring inside the cylinder of an internal combustion engine. The computer program has been calibrated by using the measured results at sea-level.An empirical correlation has been proposed for the correction factor of the engine power in the range of 100 to 44 kPa, where the low pressure corresponds to a standard altitude of 21,000 ft (7 km). The present study suggests that the correction factor is proportional to the ambient pressure raised to a power which depends on the engine speed (between 1 for low and 2 for high engine speed). Previous studies have suggested a power of 9/8 (1.125), their validity, however, was limited to low engine speeds and a minimum ambient pressure of 70 kPa which corresponds to an altitude level of up to 10,000 ft (3 km).