Composting process depends on microbiological decomposition of organic matter in oxygenic conditions proceeded by the thermopile microorganisms (bacteries) and moulds. During composting process, which thermopile phase can last from 4 to 7 weeks, there is a lot of heat energy emission. Especially high level of it is received in initial phase - because the temperature inside the composted pile can reach 60-75°C and stays on the level over 50°C for a dozen or even few dozen days. In order to carry out the laboratory experiments, which will fulfill the field conditions in 2002, at the Institute of Agricultural Engineering a bioreactor for the study of the organic material decomposition was constructed. Because of thermal isolation from outside conditions and complex sensor system, bioreactor is perfect experimental set-up, which allows to accumulate a large database. This fact makes possible to create composting model with placing emphasis on heat emission. As a modeling tool, the artificial neural network was used.