AbstractPrevious studies show that fast magnetosonic (MS) wave with frequency of tens of Hz may propagate to low L‐shells (L < 2.0) in the inner plasmasphere and mode convert to electromagnetic ion cyclotron (EMIC) wave where its wave frequency is close to the local crossover‐cutoff‐resonance frequency triplet through different propagation paths. In this study, we investigate how this mode conversion process may depend on the ion composition and electron density by full‐wave simulation with various plasma density profiles and initial wave normal angles (WNAs) of incoming MS wave from magnetic equator and a higher latitude of 15°. We find that critical fraction of minority ion (He+ or M/Q = 2 ion − D+ or He++) exists during the mode conversion, over which the conversion from MS wave to H+ band right‐handedly polarized EMIC wave starts to become negligible. This critical fraction decreases with the incident WNA of MS wave from equator. The efficiency of this conversion process also decreases with the electron density. As incoming MS is from higher latitudes off equator, optimal fraction of minority ion exists, with which the efficiency of conversion from MS wave to H+ band left‐handedly polarized EMIC wave reaches a maximum. Moreover, this maximum conversion efficiency varies with the incident WNA and may approach 100% with WNA close to 90° at the magnetic latitude of 15°. We also find that localized wave structure may be formed when the converted H+ band EMIC wave is trapped between local H+ − D+(He++) bi‐ion resonance frequency and D+(He++) − He+ left‐handed polarization cut‐off frequency with a small M/Q = 2 ion fraction and a relatively larger He+ fraction as incoming MS wave is near equator.