Methane number is the parameter used to quantify knocking tendency of a combustible gas, parameter especially relevant when said gas is used as engines fuel. The present paper proposes a new method to determine on line the methane number of natural gas. The method is based on the measurement of the gas thermal conductivity and the correlation between this parameter and the methane number of natural gas. The developed thermal conductivity microsensor is ascribed to microsystems technology. It integrates two Pt thermoresistors each patterned on a microbridge, defined in turn in the silicon substrate by surface micromachining. Porous silicon is used as sacrificial layer. In the considered methane number range (60-100), a linear output with a sensitivity of 0.95mV/1MN is obtained when a 50mA constant current is applied to the sensing thermoresistor. Furthermore, microdevice is mounted in a gas line rig in order to test it in field. Additional tests are still required but it can be concluded that developed microsensor is a valid alternative to measure in situ the methane number of natural gas. Resumen publicado por el autor.