Flight dynamic simulation programs are useful to estimate dynamic performances of an aircraft and to develop flight control laws. Usually, simulation programs were developed by the causal programming method which required a distinct relationship between inputs and outputs. Recently, a new approach, acausal programming, has been developed and applied in many fields to deal with the disadvantages of causal simulation programs. The authors have developed a flight dynamic simulation program by using Modelica which is an acausal programming language. The developed program has been tested by several simulations for attitude and velocity hold control. It has been shown that servo systems for control surfaces can be designed by this program. The level of detail of models can be easily increased by using the library of Modelica. The simulation program can be used to develop unmanned aerial vehicles and rotor crafts.