Abstract: This paper describes an experimental and theoretical study for a better understanding of the working process of the single-cylinder, two-stroke internal combustion pump The work was done in 1959, when there was a wide interest in this type of pump, but much about its internal process was still not clear. In order to clear up some of the questions then existing, the .Power Laboratory of Academia Sinica undertook this investigation. As a result the nature of some of the processes, especially the water intake and the gas intake and exhaust processes, was much clearer, and many of the observed characteristics of the pump could be explained theoretically. The study included tests with structural and operational parameters as variables, and a theoretical analysis of the ideal cycle with discussion on how the real cycle differs from the ideal one. Results of this investigation indicates that for each given pumping head, there is a set of pump dimensions which will give optimum performance Further improvements of the pump proper (excluding the gas. Producer) should be directed toward achieving these optimum dimensional proportions, improving ignition and combustion, and preventing the exhaust gas from mixing with the fresh gas in the cylinder. Some basic change in the pump construction such as the use of a partition plate would be necessary to achieve the last objective.