In our present study, the phase transition behavior of gamma-alumina doped by an alumina-sol was explored using X-ray powder diffraction and transmission electron microscopy. It was found that the addition of an alumina-sol into gamma-alumina enabled the onset of the gamma- to alpha-alumina transformation at temperatures as low as 600 degrees C. This phenomenon was designated as the "sol-effect." Although the onset temperature of the phase transformation was very low, the transformation was completed at 1100 degrees C which is the same as that observed with the seeding method. Further investigation revealed that the transformation was composed of two steps. One occurs over the temperature region from 600 to 950 degrees C where a slow increase in the content of alpha-alumina is observed. The other is the region from 950 to 1100 degrees C where a rapid increase of alpha-alumina occurs due to "self-seeding." The "sol-effect" produces powders with very fine alpha-alumina particle sizes and little agglomeration.