Complex oxide nanocrystals with a spinel structure show their remarkable optical, electronic, mechanical, thermal, and magnetic properties. In this study, we present a simple yet versatile strategy to grow self-assembled epitaxial CoFe 2 O 4 nanocrystals with well-controlled size (less than 10 nm) and single orientation. CoFe 2 O 4 nanocrystals were fabricated via phase separation in a BiFeO 3 - CoF 2 O 4 ultrathin film by pulsed laser deposition. The coherent strain at the BiFeO 3 -CoF 2 O 4 interface suppressed the growth of the nanocrystals regardless of substrate temperatures. This strain also resulted in the ferromagnetic anisotropy and interesting conducting behaviors of ultrafine CFO nanocrystals.