Crystallite size variations of nanosized iron oxide particles during γ-to r-phase transformation were examined using DTA/TG, FTIR, XRD, TEM, and BET-N 2 techniques. Synthetic γ-FeOOH and Fe 3 O 4 , prepared as precursors, both converted to γ-Fe 2 O 3 and then completed the phase transformation under thermal treatment. Starting with γ-FeOOH, the γ-Fe 2 O 3 obtained grew from the original size of 5 nm to a size about 25 nm or larger, then transformed to r-Fe 2 O 3 . The r-crystallites obtained showed sizes similar to that of γ-crystallite. With continuous heating, the r-Fe 2 O 3 coarsened to a size of ∼55 nm. Identical phenomena were observed when r-Fe 2 O 3 was prepared from Fe 3 O 4 via γ-Fe 2 O 3 . The study also found that r-Fe 2 O 3 crystallites obtained by calcinations of amorphous Fe(OH) 3 will show sizes smaller than 5 nm. The decomposition temperature can be lowered to 200 °C.