In this paper we report the magnetic properties of ultrafine Ni particles. The specific magnetic moment drastically decreases with reducing the diameter d of particles when d≤15 nm. The coercivity Hc also approaches zero when d is about 15nm. The value of effective magnetic anisotropy constant is determined as K=-5.8×105 (erg/c.c.). It is found to be larger than the bulk one. The mechanism of reversal magnetization can be understood by sphere chain model. Curie temperature Tc is obviously lower for 9nm average diameter particles as compared with the bulk, this may result from the lattice contraction.