Dependence of AC magnetization losses in a YBCO bulk on the trapped magnetic field is experimentally investigated. The experimental results showed that the losses were hysteretic and that the losses in a bulk with trapped magnetic field were larger than those without trapped magnetic field. Distributions and profiles of the trapped magnetic field in the bulk were also measured before and after applying AC external magnetic field. It was observed that the trapped magnetic field was decreased by applying AC external magnetic field. From the trapped magnetic field profile, the full penetration magnetic field is estimated and the AC losses are calculated based on the Bean model. The calculated results well agree with the measured data. (C) 2002 Published by Elsevier Science B.V.