In this paper, we report an ultrafast growth of SnO2 nanocrystals directly from similar to 4 to similar to 350 nm in a hydrothermal process (250 degrees C, time >180 h). The crystal growth system is characterized by "either small or large" particle size; that is, only two differently sized SnO2 particles, either several nanometers or similar to 350 nm, coexist. During the fast growth process, SnO2 nanoparticles assembled to form densely aggregated aggregates that can quickly transform to big (bulk-like) crystals. The kinetic analysis, for the first time, point outs that the fast growth reaction (from nanocrystal aggregates to bulk-like crystals) follows a first-order reaction law with a very large kinetic pre-exponent factor (1.53 x 10(27) h(-1)), which is in line with our proposed aggregation-induced fast crystal growth mechanism. Small-angle X-ray scattering (SAXS) study of the aggregates supports that the onset of the fast growth is closely related to an increase in the aggregation degree of the aggregates. Moreover, disintegrating the aggregation state via introduction of other particles (Al2O3) into the system prohibited the fast growth. The finding in this work provides new threads for syntheses of novel nanomaterials that may possess properties not readily obtained via conventional crystal growth routes.