Abstract: In this paper is extended the nonlinear T-Z Curve of single pile,to rigid group piles. The nonlinear T-Z curve of rigid group piles is derived from the increasing settlement of the pile shaft induced by the interaction ofpile-soil-pile system. Conclusions are:①The interaction of pile-soil-pile system obviously reduces the pilestiffness. However,it raises the ultimate bearing capacity of rigid group piles,This means that the usage of rigidgroup piles is more safe than that of a single pile,which has a big cross-sectional area.②Generally speakillg,the predicted results from the nonlinear T-Z curves in much more conservative than the observed one.③It is worthwhile to point out that the failue ratio R_f used influenced the prediction,The ultimate bearing capacity of group pilesis decresaed by 5% when the R_f is increased by 0.05.In this investigation,R_f=0.9 is used as suggested by Kraft et al.,while R_f for other soil condition needs further discussion.