This paper presents a new approach to predict temperature and heating load of a building with slabs of reinforced concrete under intermittent heating by applying the lumped constant system. The average temperature of the slabs (floors and walls) is used to evaluating the temperature of the building by single value, since the air temperature after heating stopped and the heat load are dependent upon this temperature. In this method, two time constants connected to the heating and cooling processes on slabs are used as a basis for predicting the temperature and heating load. The applicability of this method was tested by computer simulation.