It is necessary to decide the fuel compressibility, the specific weight, the sonic velocity, the flow coefficient and other physical characteristics when simulating a fuel injection system. Until the present, it would seem that the relationship between these physical characteristics and the primary factors of these characteristics has been unclear. For this reason, proper application of these characteristics could not be made. Therefore in this report, we measured the physical characteristics of the diesel gas oil and also of the fuel injection system used in this study. Moreover, after comparing the calculated values with the actual experimental values, a method of calculating the time interval for computation and the judgement values for permissible error, etc. were analyzed and were appropriately established. By using the methods described above, the results of the calculation matched adequately with the specific experimental factors, even when a void condition was present.