Conductivity is an important thermal property which governs heat transfer in a solid medium. Generally, the determination of conductivity in concrete is very difficult, because concrete is a heterogeneous material composed of cement, water, aggregate, et cetera and time dependent material of which properties change with curing age. In this study, influencing factors on thermal conductivity of concrete are quantitatively investigated by QTM-D3, a conductivity tester developed in Japan. Then, a prediction equation of thermal conductivity of concrete is suggested from the regression analysis of test results. To consider the factors influencing thermal conductivity of concrete, mortar, and cement paste, seven testing variables (age, amount of cement, types of admixtures, amount of coarse aggregate, fine aggregate ratio, temperature, and humidity condition) of the specimens are used. According to the experimental results, the amount of coarse aggregate and humidity condition of specimen are the main factors affecting the conductivity of concrete. Meanwhile, the conductivity of mortar and cement paste is strongly affected by the amount of cement and types of admixtures. However, the curing age has minor effect on the conductivity variation. Finally, the prediction formula of concrete conductivity as a function of aggregate amount, fine aggregate ratio, specimen temperature, and humidity condition is developed.