Carbon encapsulated iron carbide nanoparticles were synthesized from ferrocene in a quite narrow temperature range (about 600–800°C) under high (4 GPa) static pressure. The material produced under optimum parameters consists of the round carbide particles (5–40 nm) covered with onion-like carbon shells that are embedded into the carbon matrix. The transformation of ferrocene to nanomaterial was investigated with the X-ray and transmission electron microscopy techniques. Our results show that in the presence of an excessive carbon a solid-state Fe3C → Fe7C3 transformation is possible in nanoparticles under high (13 GPa) pressure and temperature (1200°C).