Experiments were conducted in a closed vessel using Schlieren system to study the cellular instabilities of hydrogen-air premixed flames at different equivalence ratios (from 0.6 to 2.5), initial temperatures (from 300 K to 450 K), and initial pressures (from 0.1 MPa to 0.3 MPa). The cellular instabilities of hydrogen-air flames were interpreted and evaluated in the viewpoint of the diffusive-thermal and hydrodynamic instabilities. Also, critical flame radius and critical Peclet number were measured. The results showed that for lean hydrogen-air flames, the cellular instabilities are dominantly influenced by the diffusive-thermal instability; for stoichiometric and rich hydrogen-air flames, the cellular instabilities are just influenced by the hydrodynamic instability. Critical flame radius is increased with the increase of equivalence ratio and/or initial temperature, and is decreased with the increase of initial pressure. Critical Peclet number is increased with the increase of equivalence ratio, and is insensitive to initial temperature and initial pressure. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.