The magnetic phase transition of bulk CocMg1-cCl2 and stage-2 CocMg1-cCl2 GIC has been studied by dc and ac magnetic susceptibility, and low field SQUID magnetization measurements. The critical temperature decreases with the dilution of nonmagnetic Mg2+ ions and reduces to zero at the percolation threshold c(p) : c(p) approximate to 0.5 for stage-2 CocMg1-cCl2 GIC and c(p) approximate to 0.36 for Co(c)Mg1-cCl(2). A possible one-dimensional character of stage-2 CocMg1-cCl2 GIC near c = cp and the irreversible effect of magnetization below the critical temperature for stage-2 CocMg1-cCl2 GIC (0.74 less than or equal to c less than or equal to 1) are discussed.