Pneumatic control systems are used widely in industrial fields, e.g. sequential and continuous control systems, servomechanisms, etc. The most troublesome problem of pneumatic systems, especially with pneumatic cylinders, is stick-slip which is often observed when a small signal is applied. This paper deals with the theoretical analysis of critical conditions of stick-slip. At first, generalized mathematical equations of pneumatic cylinder were estabilished with a typical friction load, and converted to non-dimensional form. Then, these equations were solved in phase space, in terms of piston velocity［U］, cylinder pressure［P］relation of slideway. Finally, examples of analysis are shown, e.g. the range of stick-slip occurance is represented graphically, and the conditions under which stick-slip occurs or does not occur are given in U-P plane.