This paper describes detailed studies of the easy axis alignment of L1₀-FePt nanoparticles synthesized by the recently developed "SiO₂-nanoreactor" method (Appl. Phys. Lett. 87 032503 (2005)). The L1₀-FePt nanoparticles can be made dispersible in styrene solution containing small amounts of oleic acid and oleyl amine. The hysteresis loop of the L1₀-FePt nanoparticles solution showed superparamagnetic-like behavior at room temperature due to the free motions of nanoparticles, indicating that the easy axis of the dispersed nanoparticles can be aligned by an application of external magnetic field. The easy axis of the L1₀-FePt nanoparticles was aligned and fixed parallel to the direction of the external magnetic field by performing free radical polymerization of styrene at 60 °C under an external field of 50 kOe. Powder X-ray diffraction measurements revealed that the L1₀-FePt nanoparticles/polystyrene composite thus obtained has a (001) preferred orientation showing a strongly enhanced (001) superlattice peak and a (002) fundamental peak, as well as a (003) superlattice peak. The composite showed strongly anisotropic magnetic properties with the room temperature coercivities perpendicular and parallel to the aligning field direction of 3.3 and 25 kOe, respectively. The anisotropy field of the composite was determined to be about 70 kOe at room temperature.