Magnetic anisotropies of single crystal ultrathin Fe ₃ O ₄ films on GaAs(100) have been studied by ferromagnetic resonance (FMR). The dependence of the FMR fields on the field orientation was measured both in plane and out of plane. The ultrathin films show predominantly an in-plane uniaxial magnetic anisotropy with the easy axis along the [0-11] direction of the GaAs substrate. The in-plane uniaxial anisotropy constant decreases with increasing thickness and changes from 7.1×10⁴ to 2.1×10⁴ ergs / cm ³ when the thickness t _Fe varies from 4 to 8 nm . An in-plane fourfold anisotropy due to cubic magnetocrystalline anisotropy coexists with the uniaxial magnetic anisotropy and increases with increasing film thickness. For t _Fe =8 nm , the cubic anisotropy constant K₁ reaches -8.4×10⁴ erg / cm ³ , which is 76% of the value of bulk spinel Fe ₃ O ₄ . The out-of-plane measurements indicate a negative perpendicular anisotropy in these ultrathin Fe ₃ O ₄ films.