In low speed two stroke diesel engines, although operating conditions, such as increase in engine rated output power and deterioration of fuel quality, are severe, reservation of reliability is indispensable. Especially the scuffing problem of the cylinder liners and the piston rings under severe running condition is the most important subject, and various countermeasures, such as improvement of cylinder liner material, sliding surface finish and piston ring coating and optimization of the cylinder liner wall temperature, are being taken. On the other hand, if scuffing of the cylinder liners and the piston rings could be discovered in an early stage, serious wear of the cylinders and the piston rings leading to the unexpected overhaul may be avoidable with adjustment of engine load and/or feed rate of cylinder oil and so on. In a ship in service where scuffing was experienced, engine data such as liner wall temperature was measured. At the same time, onboard analysis of cylinder drain oil was made monitoring of iron content in cylinder drain oil could detect the scuffing in an earlier stage compared with monitoring of liner temperature. As the result of measuring the iron content in the cylinder drain oil over a long period of time in a ship in service, it turns out that the iron content rises one or two days before the temperature rise of the cylinder liner. Therefore, it is considered that monitoring of the iron content in the drain oil is effective in the early detection of the scuffing. As the result of the engine test, the liner surface temperature was higher in case of burning the poor quality fuel that showed longer ignition delay and combustion period than in case of the Diesel fuel. This means that the liner surface is exposed more severely by the combustion flame that possibly leads to destruction of the oil film on the liner surface. Therefore, use of poor quality fuel oil is considered to be one of the causes of the scuffing. Shop test results showed minor influence of the scavenging drain water on the scuffing as long as it lasted for several hours. Results with the engine test and experience with an engine in service showed that catalytic fines influence strongly on the iron content in the cylinder drain oil. Initial scuffing was observed by the microscopic surface inspection.