The Solid solution of A $l_2$ $O_3$ into ZnO can be defined by the substitution reaction of Al$^$3+/ ions into the Zn$^$2+/ sites of ZnO crystal lattice, the tetrahedral interstices composed of four neighbor oxygen ions in the wurtzite structure. Since the reaction either creates new zinc vacancies or consumes the oxygen vacancies, it should be in equilibrium with ZnO nonstoichiometry and disorder reactions. The relationships make oxygen partial pressure P$sub$o2/ control the concentrations of the vacancies and consequently limit the Al solubility in ZnO, [Al$sub$zn/]$sub$max/. This paper firstly reports with a refined model for defect quilibria in ZnO that the solubility decrease with the increase of P$sub$o2/, [Al$sub$zn/]$sub$max/ P$sub$o2/$^$-1/4/.