This study examines the mechanism of the stiffness degrading by fatigue tests of reinforced concrete deck slab specimens using cyclic wheel loading machine. The examination is executed by using the specimens with chloride induced deterioration and without its deterioration. The mechanism of the stiffness degrading is investigated from the flexural displacement of slab and the strain of reinforcing bars. To obtain the stiffness degrading and the relation to the vibration characteristics of the slab specimens, the vibration test is executed at each stage until the fatigue fracture. As the experimental results, it is confirmed the stiffness degrading of the slab is appreciable from the natural frequency measured by the vibration tests.