Superparamagnetic properties of the assembly of carbon encapsulated Ni nanoparticles (Ni(C) nanoparticles) with an average particle size of 10.5 nm are studied, the blocking temperature (T-B) is determined to around 115 K at 1000 Gs applied field. Above T-B, the magnetization M(H, T) can be described by the standard Langeuin function L using the relation M/M-S (T = 0) = coth(muH/kT) - kT/muH. Magnetization measurements suggest, this assembly of carbon encapsulated Ni nanoparticles have been exhibited typical single-domain, field-dependent superparamagnetic relaxation properties. (C) 2002 Elsevier Science B.V. All rights reserved.