The quest for ultra-sensitive low-cost miniaturized gas sensors in the past few decades has sparked interest to seek alternative approaches other than the conventional gas sensors that need large surface areas and special chemicals for functionalization. MEMS thermal conductivity based gas sensors [1, 2] have been shown to be among the promising candidates since they do not rely on gas absorption or chemical reactions. These sensors show long lifetime and great stability compared to conventional gas sensor. The thermal conductivity based gas sensors rely on the resistance variation of the heated structures due to gas exposure . Typical values of resistance changes are less than few percent. Here, we present a thermal conductivity based gas sensor relying on frequency shifts of an electrothermally heated bridge operated near the buckling point, which leads to ultra-high sensitivity.