Porous mullite ceramic membranes were prepared via in-situ synthesis method using abundant natural minerals including coarse bauxite and fine kaolin powders. Thermal sintering behavior of the green sample shows that a unique dimension expansion appears during the sintering process due to the anisotropic growth of mullite crystals. The XRD analysis suggests the ceramic membrane is mainly composed of mullite phase when sintering above 1450. °C. The SEM images confirm that with an increase in sintering temperature, mullitization reaction firstly occurs in kaolin agglomerations, and then bauxite-derived alumina reacts with silica-containing phase by a solution-precipitation mechanism to form an interlocked framework of elongated secondary mullite. Both maximum and average pore sizes increase with increasing sintering temperature. The growth of mullite whiskers has a positive impact on the improvement of porosity and mechanical strength of ceramic membranes. Even after sintering at 1500. °C, the sample shows the highest open porosity of 31.6% and good biaxial flexural strength of 100.2. MPa.