In direct-injection engines, combustion and emission formation is strongly affected by injection quality. Injection quality is related to mass-flow rate shape, momentum rate shape, stability of pulses as well as mechanical and hydraulic delays associated with fuel injection. Finding these injector characteristics aids the interpretation of engine experiments and design of new injection strategies. The goal of this study is to investigate the rate of momentum for the single and post injections for high-pressure direct-injection natural gas injectors. The momentum measurement method has been used before to study momentum rate of injection for single and split injections for diesel sprays. In this paper, a method of momentum measurement for gas injections is developed in order to present transient momentum rate shape during injection timing. In this method, a gas jet impinges perpendicularly on a pressure transducer surface. A range of parameters including gas pressure and injection duration is considered in the experiments at a constant back pressure to validate the method. The method also allows us to find mechanical, electrical and hydraulic delays of the injector. Split injections have been studied using this method and it is found that pressure reflection problems found in the conventional rate tube testing, reported in literature, does not exist in this momentum measurement method. The results show that smaller pulses with lower injection pressures have higher coefficient of variation.