Most of recent studies on strengthening material and method for concrete members have focused only on beams but rarely on slabs. In this study, an experiment was carried out on bridge decks strengthened with isotropic carbon fiber sheets and grid typed carbon plastic, under fatigue loads. The behavior was compared to the fatigue behavior of non-strengthened bridge decks. Test results show that the strengthened bridge decks not only improved in fatigue strength, but also in load-displacement relationship and total released energy as opposed to the non-strengthened bridge decks. A fundamental strengthening design procedure for the deteriorated concrete bridge deck considering fatigue behavior is also proposed in this study.