Under high-pressure conditions such as those existing in a common-rail system, a serious concern is the possibility of wax precipitation from diesel fuels and the consequent deterioration in the diesel engine performance, even if the temperature is higher than the CP (cloud point) of the fuel. By using a diesel engine with a common-rail system, high-acceleration tests were performed under high-load conditions for three test fuels, which correspond to Japanese JIS grade 2 diesel fuels but have different cold-temperature properties, at a temperature exceeding their CPs. The test revealed differences in the engine speed and the throttle position among the test fuels. It was suggested that there was a possibility of a fault in fuel injection caused by the wax precipitation at high pressure. In order to clarify the cause of this phenomenon, the wax precipitation pressure of the test fuels were measured at each temperature by using a wax precipitation measurement system based on an optical technique. The results indicated that the wax precipitation occurred in the fuels at a temperature of 20°C higher than the CP of the fuels under the operation pressure (approximately 130 MPa) of the common-rail system that was used for conducting the engine tests in this study. The wax precipitation pressure of market diesel fuels and model fuels were then measured at each temperature in order to clarify the relation between the wax precipitation behavior and fuel composition. The results revealed that light n-paraffins, isoparaffins, and alkyl benzenes offered an advantage in preventing wax precipitation from diesel fuels. On the other hand, mono-naphthenes, which were effective in lowering the CP, were less effective in preventing wax precipitation at high pressure.