The moderator temperature coefficients of reactivity for a pressure-tube-type reactor consisting of highly heterogeneous cells with D2O moderator, H2O coolant, and 28-pin fuel cluster contained in a pressure-tube have been measured in order to clarify their dependence on the moderator temperature. A new experimental method has been developed that is applicable to ordinary critical assemblies not equipped with any special temperature control system. In this method, temperature changes of the core, which is subjected to natural cooling that permits heat transfer from the moderator to the H2O coolant, are measured continually, together with the resultant changes in reactivity. The data are analyzed with use of least-squares fitting to nonlinear functions. It has been clarified that the moderator temperature coefficient of reactivity of this type of reactor is markedly dependent on the moderator temperature, decreasing with increase in the temperature. In a clean lattice of 1.2 wt% 235U enriched UO2 fuel that contains no 10B in the moderator, the coefficient changes its sign from positive to negative at ∼40°C. Addition of 3.9 ppm of 10B into the D2O moderator of the same lattice causes the value of the temperature coefficient to shift considerably toward the positive side. Calculational results by the WIMS code reproduce quite well the experimental values of the temperature dependence of the moderator temperature coefficient of reactivity. However, their absolute values are rather small compared to the experimental ones.