Acoustic emission is a very important test among non-destructive tests, and it has been applied for the detection of failures in various types of equipment in the petroleum industry, such as pressure vessels, tanks and pipelines. In this study, the effect of pre-existing uniform corrosion on the corrosion fatigue specimen and smooth specimen on fatigue behaviour of API 5L X70 steel pipeline in long life range was investigated by using acoustic emission in fatigue test. The presence of pre-existing uniform corrosion specimen, produces by 4 month immersion in sulphuric acid, H2SO4 (concentration 0.2 ml H2SO4 in 5000 ml deionized water ). Fully reverse (R = -1) constant amplitude fatigue test were done in corrosion fatigue and smooth specimen in different stress amplitudes. It was found that, effect of pre-existing uniform corrosion significantly reduced the fatigue life of the steel pipeline. The correlations of accumulative AE counts for corrosion fatigue higher than fatigue test because of the mechanisms of possible AE sources due to corrosion.