Methanol has many advantages as a fuel for automobile use fuel cells compared with hydrogen. There are two types of fuel cell systems using methanol. One is methanol reform type, and the other is direct methanol type. The direct methanol fuel cell system consists of simple and compact equipments, and suited for automobile use. This research analyzed characteristics of power output and thermal efficiency in a direct methanol fuel cell using a newly developed methanol concentration measuring system. Influences of membrane thickness, cell temperature, and methanol solution concentration on power output and thermal efficiency were analyzed.