[en] As some approach to the optimization of pumping power in a gas-cooled reactor, thermo-hydraulic analysis was made for an annular passage model. This analysis is associated with the problem of realizing axially uniform temperature distribution within a fuel rod. In the first part, some general thermo-hydraulic consideration with a straight double tube model is given, and it is shown that the pumping power can be represented by Reynolds number and diametral ratio, if physical constants, the outer diameter of inner tube (fuel), and the length of the tube are fixed. In the second part, specific analysis was made for three different distributions of heat flux: uniform distribution, sine distribution, and pseudo-sine distribution. Finally, the results of numerical computation for these distributions are presented, and it is concluded that there exists an optimum diametral ratio that minimizes the pumping power. This situation is associated with the combined effect of the increase of Reynolds number and the decrease of pressure loss, when the diametral ratio is increased. (Aoki, K.)