Friction directly affects the motion quality and performance of a pneumatic cylinder system. The frictional behavior of pneumatic cylinder depends on pressure, velocity, diameter, lubrication, and working environment, etc. The relative humidity of the atmospheric environment always affects the friction of dynamic parts. The presence of different humid films on the surface of dynamic parts will cause a change in the frictional behavior of the pneumatic cylinder, which is expressed as the difference between static and dynamic friction. This paper presents the results of research on frictional behavior of a pneumatic cylinder when the atmospheric humidity changes. The studies were conducted at relative humidity of 51 %, 75 % and 99 %, with velocities of 5, 10, 30, 50 and 100 mm/s, respectively. The results show that the effect of atmospheric humidity on the static friction force (FS), the dynamic friction force (FD) and the difference between them is clear. The static friction and dynamic friction forces decrease when the humidity increases at the studied velocities. The change in dynamic friction force is 1.5 times greater than static friction. The difference between static and dynamic friction is large at low speeds and low relative humidity, and vice versa. Moreover, it also shows that the stick-slip phenomenon is improved as the relative humidity increases.