Servo pneumatics is a mechatronic approach that enables accurate control of pneumatic drives which are widely used in automated systems. This technology can provide an alternative for less efficient electromechanical and high cost hydraulic systems. The main problem in the servo pneumatics is the fact that the variables associated with the system such as frictional forces, mass flow rate and pressure in the chambers, varies in nonlinear fashion. The paper presents nonlinear mathematical model for motion of cylinder, pressure in the cylinder chambers and mass flow rate from valve to cylinder are derived from basic gas laws and dynamics of the system. The equation for friction force based on LuGre model is also presented in the paper. Further, a nonlinear state space representation for the servo pneumatic positioning system is obtained. The developed model can be used to simulate the system response, which will help in choice of components used and also in the development of precise controller for the system. © (2013) Trans Tech Publications, Switzerland.