The use of numerical models and simulation during development, testing and operation of fuel injection systems is becoming more and more an important tool as modern computer capability increase rapidly. Numerical simulations support the engineer in understanding his system better, and enables him to make modifications and test the response within seconds. Advanced fuel injection systems consist of components from several different energy domains such as hydraulics, mechanics and electronics, working together in a highly dynamic process. Despite the latest development of powerful numerical methods and computer capability, the preparation of a mathematical model in the proper form is not a trivial task, particularly when the system is nonlinear and involves more than one energy domain as for the fuel injection systems. Bond Graph modeling has proven to handle these multi domain systems efficiently and consistently in a unified approach. The purpose of this paper is therefore to illustrate the use of the bond graph method during development and modification of numerical fuel injection models. Reusable basic component models in a hierarchic model library has been an important objective, rather than building a model of one specific system. An example is used to illustrate the method, and simulation results are compared with data obtained from experiments.