Fatigue cracks may initiate and propagate at welded joints of marine structures due to repeated wave loading. When fatigue cracks are detected in service condition, it is preferable to have an evaluation tool to determine the necessary countermeasure action with minimum intervention to the operation. For this purpose, the authors have been developing a prediction system of fatigue crack propagation, "CP-System". In the present paper, the crack opening and closing simulation is applied to structural components in welding residual stress so that fatigue crack propagation can be reasonably estimated for welded structures under variable amplitude loading. In order to confirm the applicability of the proposed method, fatigue tests using base metal specimen and welded joint specimen are carried out. The predictions were found to be in good agreement with various fatigue test results including variable amplitude tests.%船舶・海洋構造物は長期間の供用に伴う繰返し荷重によ り、溶接部を起点として疲労き裂が発生・伝播することが ある。供用中に疲労き裂が発見された場合、修理の緊急性 を判断し運用計画への影響を最小限におさえるための合理 的な評価ツールが必要である。そこで著者らは既存のFEM モデルを流用し、構造物に生じたき裂の進展挙動（進展経路 および伝播寿命）をFEMの繰返し計算により推定する手法 （以下CP-Systemと称する）を提案した。
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