This paper describes how to calculate stiffness reliability for bus body frame in the specified case. The principle of the Response Surface Method and solving process of reliability calculation are presented. Designing response surface by central index method and solving undetermined coefficients of unknown function by interpolation technique, the fitted response surface placed on check point is founded. Then failure probability of bus frame is got by Monto Carlo method and corresponding reliability is obtained. The calculation results of a specified bus body show that stiffness reliability of frame is 100% and it is necessary that the structure should be improved because of strong stiffness.