Performance of the LNG (liquefied natural gas) engine was tested on overall conditions at four different coolant temperatures, the temperatures were set at 60℃, 70℃, 80℃ and 90℃ in the tests. The testing results showed that coolant temperature influenced on performance of the LNG engine, the higher coolant temperature was, the better fuel economy would be, but NOxemission would be more. The coolant temperature also influenced on HC and CO emissions, but the influence was not obvious. By doing the tests, the optimum coolant temperature MAP of the engine was obtained.