The tendency for coiled tubing to grow in diameter and thin in wall under a combination loading of internal pressure and cyclic bending. This can occur in spite of the fact that nominal stresses due to internal pressure loading are well within elastic limits in both hoop and radial directions. The deformation mechanisms are described. Fatigue tests of 10 coiled tubing specimens are finished. The results show that the trends in coiled tubing deformation behavior are according with the context of theory.