Corrosion is a term defined as oxidation of a metal, which is observed in many parts. A structure is subjected to fatigue loading when a certain load is applied repeatedly. If this structure which was subjected to prior corrosion, under fatigue load, it reaches destruction far faster than that subjected in an inert environment. The aim of this work is to study the effect of the prior corrosion on fatigue behavior of steel alloy CK35 under constant amplitude stress. The NaCl solution used in this investigation is a 3.5wt% mass mixture of sodium chloride (NaCl) salt and distilled water corresponding approximately to the composition of sea water. The samples are immersed in the solutions for 80 days on the solution that replaces every eight days to maintain the concentration of the solution. The results observe that the fatigue life decreases in different percentage at different constant stress amplitude loading. The highest decreasing life factor is in range (9-29.3%) at amplitude stress in range (77-62% of𝜎 𝑦 ) while the decreasing life factor is in range (68.7-40.2%) at amplitude stress in range (46-31% of𝜎 𝑦 ). Also the experimental results show that prior corrosion for 80 days reduce the fatigue limit of steel alloy CK35 for 13.229%.