The purpose of this study is to compare fuel spray characteristics from mechanical and common-rail systems quantitatively for retrofitting mechanical diesel cars. Outdated diesel cars with mechanical diesel engines emit pollutant emissions and have low fuel efficiency. One of the strategies to retrofit mechanical diesel engine is to adopt a common-rail fuel-supplying system. This strategy has the advantage that it is relatively simple because it only requires the fuel injection system to be changed to a common-rail system, and the other components remain as they are. To consider applying a common-rail system, its effects should be confirmed by comparing the spray characteristics between the mechanical and the common-rail injection system. In this study, we investigated spray visualization, injection rate, and phase Doppler particle analyzer through experiments, and we compared the spray atomization results quantitatively. The results revealed that the spray development of the common-rail injection system is faster than that of a mechanical system using the same injection quantity; also, the spray of the common-rail injection system has a low Sauter mean diameter, and the common-rail system has the added advantage that it provides the ability to independently control various injection conditions, such as adjusting the injection rate by changing injection pressure and energizing time.