theta- to alpha-phase transformation of three different nano-sized theta-Al2O3 powders pre-treated with high uniaxial pressure (250-750 MPa) was examined. During phase transformation, the presence of critical crystallite sizes of theta-(d(c)theta = similar to 25 nm) and alpha-Al2O3 (d(c)alpha = similar to 17 nm) and the primary size of alpha-Al2O3 (d(p) = similar to 45 nm) is required. The high pressure induces a removal of the agglomerate state among the theta-Al2O3 crystallites and an increase in bulk density of the green compact. The former eventually creates homogeneity in the interspacing of the theta-particles (= crystallites) that results in a narrowed exothermic peak on the differential thermal analysis (DTA) profile while the latter brings the particles closer in distance, thus resulting in a reduction of the transformation temperature. And the de-agglomeration effect presumably abates the finger growth of alpha-Al2O3 crystallites. The estimated lowest temperature for the completion of theta- to alpha-Al2O3 phase transformation using theta-powders derived from boehmite is 1050 degreesC at a heating rate of 10degreesC/min. And the process duration is about 3.5 min or 35degreesC on the DTA profile. (C) 2002 Elsevier Science B.V. All rights reserved.