This paper reports on studies of cellular instability of lean hydrogen/air laminar premixed flames with an equivalence ratio of 0.6 at a 5 atm and a 25 atm pressure. Numerical simulations employing a detailed chemical kinetics mechanism and detailed transport properties are carried out to simulate the initial linear growth of instability as well as the nonlinear evolution of flames. At the initial linear growth stage, the amplitude of the initial sinusoidal shaped flame front grows exponentially. Later on, in the nonlinear evolution stage the flame front develops into a cellular surface with wavelengths and amplitudes different from its initial ones. At higher pressures, hydrodynamic instability is enhanced, due to smaller flame thermal thickness; the flame fronts are more chaotic. Chaotic flame fronts are captured during the nonlinear evolution stage and it is shown that the evolution is very sensitive to initial perturbations. Two phenomena in the nonlinear evolution process are observed, mode-lock and preferential choice of modes. Both of these appear in connection with the initial disturbances to the flame front. Sensitivity of the numerical results to numerical schemes and the computational setups is examined. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.