A new method based on the nonlinear flux diffusion model was used to study flux pinning and critical current density of Ag - Bi-2223 tape. Complex a.c. susceptibility was measured systematically as a function of temperature T, a.c. field amplitude h, frequency f and d.c. field . The dependence of effective barriers U on T, and current density J were determined to be , where and . The results are consistent with those determined by other experiments. as a function of T and , , is estimated and compared with those measured directly by the transport four-probe method. It is therefore shown that the a.c. susceptibility technique is also a powerful tool for the study of Ag - Bi tape samples.