The precipitate of FeCl2.4H2O and NaOH, Fe(OH)2 was rapidly made to oxidize by H2O2 to prepare $delta$-FeOOH. The particle size, surface and morphology of $delta$-FeOOH, and the shape and structure of thermally decomposed $delta$-FeOOH were investigated by the use of high resolution STEM. $delta$-FeOOH prepared under the condition of reaction temperature of Fe(OH)2 at 4$0^{circ}C$, [OH-][Fe2+]=5 and aging time of 2 hr Fe(OH)2, had 630$AA$ mean particle size, 4~5 aspect ratio, 20.8 emu/g saturation magnetization and 210 Oe coercivity. The edges of $delta$-FeOOH were inclined to (001) about 41$^{circ}$, 60$^{circ}$ and coincident with (102), (101) respectively. When $delta$-FeOOH was thermally decomposed at 25$0^{circ}C$ for 2 hr in vacuo, which had micropores of 0.9 nm thickness and crystallites of 2.4 nm thickness. (001)hex, [10]hex. of $delta$-FeOOH parallel with (001)hex, [100]hex. of $alpha$-Fe2O3 respectively. This showed three dimensional topotaxial structure transition, which was investigated by SADP (Selected Area Diffraction Pattern) of STEM.