Cooling phenomenon of finned-wall cylinder has been studied in an operating engine. This analysis is based on the concept that the cylinder wall is considered to be one part of the engine thermal system. That is, by cycle simulation, the heat transfer rate from gas to metals was first calculated, and then the temperature distributions in the cylinder fin were obtained by a finite element method. In the meantime, the squish effect of the hemisphere chamber is incorporated into the simulation. The temperatures could be measured continuously by means of thermocouples located at 10 measuring points in the cylinder fin of the test engine of two-stroke cycle. The results showed that the cylinder temperatures increased with increasing engine speed, and also with increasing squish ratio due to increased heat loss. In this way the agreement between the calculated and the experimental results could be checked.