Examination on the existence of the critical and primary crystallite size (d c r ∼ 17 nm and d p ) 50−55 nm) phenomena during θ-to r-phase transformation of the nanosized Al 2 O 3 powder was designed and conducted using two θ-Al 2 O 3 powders of different crystallite sizes. Due to the critical size, the size dependence of the transformation temperature, the rate of transformation, and the inevitable presence of residual θ-phase in the thermal-treated and re-treated "100%-transformed" sample can be expected. Due to the two characteristic sizes and the formation of primary crystallites by the coalescence growth of the critical crystallites, the variation in mean r-size during the transformation can be predicted by means of the difference in the growth rate of the two θ-powder (different crystallite sizes) systems associated with proper thermal treatment techniques. Further, it is important to note that the presence of primary crystallites at various thermal-treated samples becomes possible.