In order to substantiate the safe operation of fast reactor plant components at elevated temperature exceeding 500C, an evaluation of structure integrity under the presence of crack-like defects is required to complement design assessment. CRIEPI has generated the guideline for structural integrity assessment for fast reactor plants. Part II of this guideline concerns flaw evaluation, and provides a simplified J-evaluation method to predict creep-fatigue crack propagation behavior. The revision of the guideline is currently under way to improve the predictability of the method. This paper gives an outline of the modification of the simplified J-estimation method using reference stress, including the consideration of elastic follow-up.