The diffusive-thermal instability of premixed flames under the low temperature environment was studied by unsteady calculations of reactive flows based on the model equation. As the unburned-gas temperature became lower, the growth rate decreased and the unstable range narrowed at Lewis numbers lower than unity, which was due to the decrease of the burning velocity of a planar flame. As for the growth rate and unstable range normalized by the burning velocity of a planar flame, the former increased and the latter widened. This was due to the enlargement of Zeldovich numbers. Owing to diffusive-thermal instability, cellular-shaped flame fronts formed. The normalized burning velocity of a cellular flame increased as the unburned-gas temperature became lower.