Flux modulation machines (FMMs) employ unmatched pole-pair configurations between the excitation and the armature units, achieving high torque density through magnetic field conversion. However, their core loss mechanisms remain insufficiently studied. This paper investigates iron loss optimization in an 18-slot/28-pole interior permanent magnet flux modulation machine (IPM-FMM) using finite element method. A parametric study focused on two key variables: pole shoe width and PM angle (α). The optimal design (6° pole shoe width, α =132°) demonstrated: (1) 10.37% reduced no-load iron losses; and (2) 8.97% lower load iron losses. These improvements validate geometric optimization for core loss reduction in IPM-FMMs. The study provides a practical framework for balancing electromagnetic performance and efficiency through structural modifications, while offering new insights into rotor geometry-loss relationships in IPM-FMMs.