The top-heat-type heat transport loop is composed of a heated section, a cooled section, a reservoir, valves and pipes connecting these components. The working fluid is circulated inside the loop by utilizing its vapor pressure, and the heat is transported downward from the heated section to the cooled section. This paper describes fundamental experiments and theoretical analyses on the operational characteristics of the loop, and discusses the temperature difference between the heat sink and the reservoir, and that between the heat source and the reservoir. In the experiments, the transient variations in temperature and pressure inside the loop were measured, and the liquid velocity was calculated from the results, thereby clarifying the fluid-flow characteristics inside the loop. Moreover, consideration is given to reducing the temperature difference between the heat sink and the reservoir. In the theoretical analyses, a mathematical model of the loop was formulated, and its validity was confirmed by comparing the numerical and experimental results. Analyses were also carried out for varying design and operation parameters of the loop in order to reduce the temperature difference between the heat source and the reservoir.