Magnetic behaviours of the mixed crystal, NipMg(1-p)(OH)2, as a randomly diluted magnet of the quasi-two dimensional antiferromagnet, Ni(OH)2, were investigated by measurements of the susceptibility and the magnetization. In pure Ni(OH)2, the three kinds of exchange interactions were estimated as follows: J1/k̃2.70 K, J2/k̃-0.28 K and J3/k̃-0.09 K. The anisotropy energy mainly comes form the crystalline anisotropy. In NipMg(1-p)(OH)2 the paramagnetic susceptibilities obeyed the Curie-Weiss law well above the critical temperature, TN(p), for the concentration, 0.4≲p≤1. Below p̃0.4, the susceptibilities obeyed the Curie-Weiss law, but with the different Curie and the Weiss constants in the lower and higher temperature regions. This may result from ferromagnetic short range order in the lower temperature region.