In order to reduce the entropy generation of premixed hydrogen/air flame in the micro combustion chamber, the combustion chamber of the old micro reactor is modified by gradually varying the diameter of the combustion chamber. Extensive numerical investigations about the entropy generation of premixed hydrogen/air flame in old and modified micro reactors are conducted under various hydrogen mass flow rates, hydrogen/air equivalence ratios, solid materials and inlet/outlet diameter ratios. Results suggest that the modified micro reactor has lower total entropy generation than that of the old micro reactor. This is attributed to that the temperature gradient of the burned gas in the modified micro reactor is lower than that in the old micro reactor after chemical reactions. Finally, the largest descent percentages of total entropy generation are achieved under various conditions, which provide significant reference values for hydrogen energy usage under micro scale combustion. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.