This article provides a detailed explanation of the working principle of diesel generator sets, describing how diesel fuel is converted into electrical energy through a series of energy transformations involving combustion, mechanical motion, and electromagnetic induction. It outlines the structural composition of a typical generator set, including the diesel engine, synchronous generator, control system, cooling components, and base frame. The article also explains the four-stroke energy conversion process inside the engine cylinder and the four stages of diesel combustion that influence engine efficiency and performance. In addition, it discusses the main characteristics, typical applications, and essential operation and maintenance practices of diesel generator sets, helping readers better understand their role as reliable power supply equipment in industrial, agricultural, communication, and public service sectors.