The relationships between the triboelectric charge to mass ratio (Q/M) and volume ratio (Q/V) of a monocomponent magnetic toner and the properties of magnetite incorporated in the toner were studied. To remove the influence of the development system on toner charging, the triboelectric charge of the magnetic toners against an iron oxide powder carrier was measured. Several kinds of magnetite differing in their particles size and shape were used, and the concentrations of magnetite in the toner were changed from approximately 5 to 55 wt%. The Q/M and the Q/V decreased proportionally with an increase in magnetite concentration. The Q/V decrease means the Q decrease for the same diameter toners. The relationships between the Q decrease and the number of magnetite particle and the total cross-sectional area of magnetite particles on a toner surface for same diameter toner group were analyzed. The result suggests that total cross-sectional area and surrounding area of magnetite on a toner surface has no triboelectric charge. In conclusion, the Q can be represented as Q = Q₀ × (1 − S_m/S), where, Q₀ is charge for the magnetite free tonerS is surface area of a toner, and S_m is the total effective cross-sectional area of magnetite on a toner surface.