This paper presents the effects of a lubricant oil droplet on the start of combustion of a fuel-air mixture. Lubricant oil is thought to be a major source of low-speed pre-ignition in highly boosted spark ignition engines. However, the phenomenon has not yet been fully understood because its unpredictability and the complexity of the mixture in the engine cylinder make analysis difficult.In this study, a single oil droplet in a combustion cylinder was considered as a means of simplifying the phenomenon. The conditions under which a single oil droplet ignites earlier than the fuel-air mixture were investigated. Tests were conducted by using a rapid compression expansion machine. A single oil droplet was introduced into the cylinder through an injector developed for this study. The ignition and the flame propagation were observed through an optical window, using a high-speed video camera.The test results showed that the ignition timing triggered by the oil droplet became earlier as the droplet diameter decreased and the temperature increased. Except under some conditions, however, ignition delay was later than that of a gasoline mixture. The early triggering of combustion followed by flame propagation required the introduction of a heated oil droplet. The probability of flame propagation initiated by the oil droplet decreased with the temperature of the droplet. Numerical simulations indicated the same tendency. The test results also showed that the probability of ignition changed with the dilution ratio and the ignitability of the mixture. It was notable that the dilution with N2 and CO2 decreased more than the fuel RON.