Passive production of synthesis gas from liquid methanol using a capillary action in a packed bed of porous particles supporting a catalyst is investigated. Heating of an upper side of the packed tube while the bottom is immersed in liquid methanol causes upward fluid flow due to capillary action enhanced by evaporation and produces the synthesis gas in the high temperature region. For an increased diameter of the bed, the partial invasion of wetted region into the core of the dried region may cause a significant reduction in reaction yield. The radial distribution of temperature and its effect on the reaction are identified from the experiment. Un-uniformity caused in the radial direction is discussed based on the physical modeling of the capillary liquid flow toward the heated wall.