This research is a part of alternative fuel studies which responses to the current energy and environment crisis. The experiment was aimed to compare the performance between an original diesel engine and a natural gas dedicated diesel engine with various compression ratios of 9.0, 9.5, 10.0 and 10.5 to 1. A Daedong 4A220 4-stroke 4-cylinder 2,197 cc was chosen as the tested engine. The results showed that the natural gas engine produced higher torque and power than that of diesel engine up to 13.07% at 2,600 rpm. Natural gas engine also showed advantages in term of thermal efficiency and specific fuel consumption which are 10.03% and 36.75% respectively. In addition, NOx emission increases as the compression ratio increases to 10.5:1. Then NOx emission begins to decrease when the compression ratio increases. The compression ratio significantly affects the hydrocarbon emission concentration while it does not show any relation with the carbon monoxide emission.