Abstract: To study the influence of pile cap thickness and pile rigidity on the rigidity of pile caps and the distribution of pile loads,three and one scaled reinforced concrete caps for nine-pile and sixteen-pile respectively were cast and tested by varying the rigidities of piles.On the basis of analyzing the test results,and taking the interaction between piles and cap into account,a new relative stiffness expression of pile-cap was presented.Contrastive analysis was made among the recommended method and the existing rigidity formulas of pile-raft/box foundation.It was indicated that the relative stiffness of pile-cap calculated by the recommended method conformed to the rigidity behavior of pile caps,and it was more reasonable to use the proposed method to judge the relative stiffness of pile-cap with large rigidity of piles than to use current the methods.Furthermore,the required thickness of pile caps which satisfied the rigidity condition could be calculated using the recommended method.