This study presents the development of dynamic models for gas injection pressure that may be implemented in the design of control systems for gas-injection units. A nonlinear dynamic model was first derived and then verified by experimental measurements. This was done by using a laboratory-built, gas-assisted injection unit. The agreement between the prediction and measurement indicates that the present nonlinear dynamic model adequately predicts the dynamic behavior of gas injection pressure during the process. Although the resulting model is useful for understanding the behavior of the process and the effects of different process variables, its complexity may cause difficulties in a real control application. Therefore, a second-order model based on the basic characteristics of the nonlinear model was proposed to approximate the gas injection pressure. In order to determine the model parameters, the algorithm of recursive least-square system identification was employed. A comparison of simulated results of an identified model with experimental data showed that the model accurately predicted the transient behavior of gas injection pressure. Consequently, this low-order model can be easily implemented into the control system design of a gas-injection unit.