Abstract: Fast magnetosonic waves observed in the Earth’s inner magnetosphere are considered to be excited through the proton Bernstein instability driven by proton velocity distributions with a positive slope along the perpendicular velocity. The present study employs linear kinetic plasma theory to investigate how the concentrations of cool background helium and oxygen ions affect the growth rate of the proton Bernstein instability driven by proton shell velocity distributions. The results demonstrate that increasing the concentrations of these heavy ions makes the unstable waves move to larger wave numbers (smaller wavelengths). At the same time, their overall growth rate increases and their harmonic numbers shift toward lower frequencies. In addition, these changes of the proton Bernstein instability are more pronounced when the concentration of background oxygen ions increases, compared with that of helium ions. Finally, the reasons for these changes are explained and discussed.