The flame propagation in N, CO2 and Ar-dild. CH4-air mixts. was modeled to calc. the burning velocity used for estn. of flammability limit and explosion index. The calcd. data agreed well with the exptl. data of H.F. Coward and G.W. Jones, 1952, and J. Glowinski et al., 2002. A correlation between the burning velocity and explosion index was developed for the CH4-air mixt. (including the O-depleted air). The correlation allows to estimate the explosion index without any measurements of the max. pressure increase rate.