Abstarct: Ratcheting is one of the fatigue failure modes. Ratcheting is a phenomenon of progressive accumulation of deformation while materials are subjected to cyclic loading, especially under stress-controlled mode with non-zero mean stress. So study of ratcheting and its effective parameters is essential to control it and improve fatigue life. Fatigue is an important parameter to design engineering components. On the other hand, polyacetal is in use as a high fatigue characteristic material. To investigate ratcheting behavior of polyacetal, some experimental tests have been drawn and done. All tests were done by an Instron servo-hydraulic machine. Effects of frequency, thickness, stress amplitude, mean stress and internal pressure on ratcheting of polyacetal tube have been examined. To predict fatigue life stress approach and strain approach were investigated and a relation to forecast fatigue life presented. Result shows stress approach is more successful than strain approach to predict fatigue life. An analytical solution to calculate Poisson's ratio on elastic and plastic region presented. Also result shows if frequency is more than a specific value the time of the test is stable. Effect of thickness was studied and shown that similar axial stress has no effect on fatigue life. Increasing in thickness, results in decreasing the limit strain.