Based on the technique proposed by Muki & Sternberg, a rigorous analytical method for calculating the interaction factor between two piles with different lengths, diameters and properties is presented. The pile group in layered soils is calculated by using the principle of superposition. The validity of the presented method has been verified through comparing with those from other existing solutions. A parametric analysis is made to study the pile group settlement and the loads shared by the individual pile. The proposed interaction factors of pile-pile can consider the strengthening effect of intervening piles. The proposed method can be used to analyze the large pile group.