In this paper, a method to calculate the natural vibration of rectangular and trapezoid stiffened shell is presented. Several numerical calculations using present method are carried out, and following conclusions are obtained. (1) The natural vibrations of rectangular stiffened shell calculated by present method have a good agreement with the experimental results. Then it can be said that the propriety of present method is confirmed. (2) The relationship between the reinforcement method of shell and natural vibration is analytically investigated. If the stiffeners are arranged to the same direction that the curveture is small, the reinforcement with stiffeners is not effective to the increment of natural frequency. On the other hand, if the stiffeners are arranged to the direction of large curveture, the natural frequencies increase by the multiplication effect of the increments of rigidity due to curveture and stiffeners. (3) The effect of the shape and magnitude of initial deflection to the natural frequencies is also investigated choosing the actual deck structure of pure car carrier as a subject of investigation. And it is seen that in the case of complex initial deflection and higher order vibration, a little initial deflection is causative of the increment of natural frequency. (4) According to the calculated results for the trapezoid stiffened shell, because of the relationship between the strain energy due to membrane force and that of bending moment, the natural frequencies decrease as upper side-lower side ratio c / a becomes large in the case of small initial deflection or small c / a . But the natural frequencies increase conversely in the case of large initial deflection and large c / a .