A catalytic combustible sensor for LPG/LNG detection was fabricated on $Al_2O_3$ substrate using planar technology. The catalysts of Pd and Pt were added to ${alpha}$- and ${gamma}-Al_2O_3$ powders. The mixture of Pt, Pd and $Al_2O_3$ were homogenized by using a three roll mixer. TCR characteristics of Pt heater were optimized with the heat treatment temperature. Sensing properties were investigated as a function of the microstructure of $Al_2O_3$, the gas concentration and the variation of input voltage. ${alpha}-Al_2O_3$ sintered at 500 $^{circ}C$ is more suitable as LPG/LNG sensor due to good grain shape and size distribution of about 300 nm than that of ${gamma}-Al_2O_3$ which is in irregular shape and with a particle size of 5-30 ${mu}m$. The sensor has shown maximum output voltage of 14 mV for 1000 ppm $C_4H_{10}$ and 3.8 mV for 1000 ppm $CH_4$ at 5.0 V input voltage.