Alumina membrane was prepared by sol-gel coating method using boehmite powder(${Upsilon}-AlOOH$). The supported and the unsupported alumina membrane were fabricated to investigate the phase transformation and change of microstructure. It is important to control the homogeneous pore size and distribution in application of filtering process. The ${ heta}-to;{alpha}-AL_2O_3$ phase transformation (XRD) and the change of microstructure was investigated using scanning electron microscopy(SEM). XRD patterns showed that the supported membrane had $100^{circ}C$ higher ${ heta}-to;{alpha}-AL_2O_3$ transformation temperature compared to the unsupported membrane. The similar effect was also observed for microstructural change of the membrane.