It is known that external magnetic field not only causes the magnetization loss in a superconducting tape, but also increases the transport current loss, by the so-called "dynamic resistance." In power applications, superconductors carrying alternating current should be in alternating magnetic field. The frequency of the field is usually the same as that of the transport current, but they have phase difference. We developed an experimental system, which can measure transport current loss in external AC magnetic fields. With this equipment, we explored the effect of external field on the transport current loss with various field strengths and phase differences. This paper describes experimental results and discussions on the effect of amplitude and phase on transport current loss of HTS (High Temperature Superconductor) tape.Index Terms-AC loss, HTS tape, magnetic field, phase difference, transport current loss.