To investigate the synthesis of $eta$-Al2O3 and its crystallization behavior by oxalate coprecipitation method, the optimum pH range for oxalate coprecipitates has been theoretically calculated from the solubility products and the equilibrium constans of each metal ionic species and their solubility diagram wa obtained. The optimum pH range for oxalate coprecipitates at room temperature was estimated as <4. In experiment, we found that the optimum condition for oxalate coprecipitates was pH<1, which was not doped with pH controller. The Na+ ions were easily exchanged for the NH4+ ions of NH4OH which was used as pH controller, and those NH4+ ions were supposed to affect the crystallization behavior of $eta$-Al2O3. The thermal decomposition of all complexes was almost complete below 40$0^{circ}C$. The primary product of the decomposition process was m-Al2O3, which transformed to $eta$"- or $eta$-Al2O3 at temperature higher than 100$0^{circ}C$. We found that the powder prepared at 120$0^{circ}C$ had only $eta$"- and $eta$-Al2O3.EX>-Al2O3.