Dynamic modeling of a diesel injection system is imperative to identify the forces applied to the system's elements and determine the conditions under which the system will be stable. The results of dynamic modeling are especially important for injection systems with different operating modes.This work uses a five-degree-of-freedom system to represent a diesel unit injector. The Bondgraph Method was used for formulating the kinematic and dynamic equations. The inputs to the system were the velocity of the cam follower and the pressure force exerted by the fuel on the plunger. The coefficients of the system's characteristic equation were then determined. Routh's criteria were considered to identify the region of stability for the unit injector.