It is pointed out that TBO of diesel engines made by different manufacturers can not be compared with each other, because when TBO is determined, there is no identical criteria, and manufacturers often have different considerations due to various reasons. The various factors which need to be considered when determining TBO are analyzed, and the principles MTU uses to determine TBO are introduced in particular. Depending on the failure rate curve, MTU determines TBO of its products as per predetermined engine load-time profile of an application group. TBO also can be adjusted as per actual load profile. On the other hand, to match TBO the preventive maintenance schedule should be strictly followed. These principles are helpful for further exploring the proper way to determine TBO of diesel engine.