Cellularity of the propagating spherical laminar flames induced by the flame instability was studied for methane and propane-air mixtures at the equivalence ratios from 0.8 to 1.3 and the initial pressure from 0.10 to 0.50MPa. The Markstein number was employed to quantify the effects of the equivalence ratio and the pressure of the mixture on the cellularity due to the flame instability. The critical Peclet number, or the critical flame radius normalized by the flame thickness, at which the cells are created over the entire laminar flame surface, decreased with decreasing the equivalence ratio for the methane mixture and decreased with increasing the equivalence ratio for the propane mixture. It decreased with increasing the initial pressure at all the equivalence ratios. It decreased as the Markstein number decreased. The flame was no longer stabilized by the flame stretch at the large Peclet number though the positive flame stretch will restrain the Darrieus-Landau flame instability. Especially with the negative Markstein number, the flame was unstable even at the small Peclet number.
