Abstract: Diesel engine is a typical tribological system.Introduces a kind of on line wear condition monitoring system for marine diesel engine. The system is capable of particle detecting,lubricant quality detecting,and shaft torque moment and transient rotation velocity detecting.It detects the wear particles in lubricant with on line ferrograph so as to show the wear condition of the tribo pairs. The vertically fixed detector is used in this system. The system detects on line the relative variation of lubricant quality with grid capacitance sensors, which consist of an upper capacitance and a lower capacitance. It can distinguish the relative variation of the dielectric constant of lubricant by pollutant such as water and metal particles whose gravity is larger than that of the oil.It detects the shaft torque moment and transient rotation velocity of the diesel engine with photoelectric sensors and corresponds the wear condition with the power condition and the change of transient rotation velocity due to burning pressure change. It is also helpful to judgement of the location of cylinder wear.