The magnetization curves are numerically studied based on nonlinear flux creep model simultaneously considering the effects of surface barrier and bulk pinning. The numerical results show that there exists a maximum of magnetization at an intermediate field ("fishtail effect" (FE)) in the M-H curves, which originates from bulk pinning. These simulation results also give a prediction that the FE in M-H curves cannot be simultaneously observed on a single M-T curve under zero-field-cooled (ZFC) process. The experimental results of melt-textured growth YBCO materials are similar with the numerical M-H curves, which suggest that the behaviors of M-H curves are the cooperation effects of the surface barrier and the bulking pinning force. Moreover, it is suggested that the surface barrier is more important at high temperatures, and the bulk pinning force plays a more important role at low temperatures. © 2006 Elsevier B.V. All rights reserved.