Dual fuel (natural gas (NG)-diesel oil (DO)) is one of the interesting strategies that can be applied to diesel engines related to emission reduction. On the other hand its application causes a decrease in diesel engine performance. So it is necessary to look for system settings that can produce the best performance. One possible setting to analyze is the NG injection pulse width. In this study, a variation of NG injection pulse width was carried out to determine the effect on the engine performance at low to high loads. From the experimental results, it can be seen that the variation of gas injection pulse width does not significantly affect torque, power, and BMEP. There is an increase by reducing injection pulse width up to 9 ms in medium and high loads but the changes are small. Variation of injection pulse width 11 ms has the lowest SFOC and the highest thermal efficiency at medium to high loads. While the biggest substitution is obtained by injection pulse width 12 ms but at 75% load, the substitution is lower than 11 ms.