The performance of a plate fin type reactor with anodized alumina catalyst for methanol decomposition was prepared and evaluated. The measured temperature difference between heating oil and reaction gas was as small as 3 degreesC and under, and it was smaller than the calculated value using the overall heat transfer coefficient obtained by inert gas and reaction heat. The response of reaction against the change of the gas flow rate and the change of the heating oil temperature were so quick that the gas temperature in the reactor and the gas flow rate of the outlet of the reactor became constant within I minute. The excellent thermal performance of the plate fin type reactor with the anodized alumina catalyst was demonstrated.