AC loss is one of the key problems in HTS applications. This study focuses on the AC loss in small Bi-2223/Ag and YBCO coils, with an emphasis on the presence of an iron core. HTS coils with an iron core can be used in a number of applications, including induction heaters, motors, fault current limiters, controllable reactors and transformers. The major concern of the study is the interaction between the iron core and the HTS coil. This study first provides a brief introduction of the resistivity-adaption algorithm (RAA) method, which is used to simulate the field penetration, based on a given flux motion theory. A general model is then proposed, with a discussion on the effect of the iron core. Results showed that a well-designed iron core is beneficial for the AC loss reduction.