Mixed municipal solid waste collected from the area of each Polish district (commune) is transferred to Regional Installations for Municipal Solid Waste Treatment. They comprise one or more of the following facilities: installations for mechanical-biological waste treatment (MBT), installations for thermal treatment of municipal solid waste, green waste composting plants and landfill sites. MBT installations have been currently the dominant technology of mixed municipal solid waste treatment in Poland. In these installations mixed waste is subjected to mechanical processes (including: crushing, separation, screening and classification) resulting in the production of the undersize fraction with usual grain size below 80mm and the oversize fraction with grain size over 80mm. Because of the necessity of stabilization and hygenization of the undersize fraction prior to landfilling, it is subjected to the process of biological treatment, e.g. biostabilization. The main aim of the research was to analyze the temperature changes during the biostabilization of the undersized fraction in thermally insulated BKB100 laboratory bioreactor. The research covered a 14-day period of the intensive phase. The analyses were performed in 6 replications. 40.1±2.2kg of waste with density of 519.2±27.5kgˑm-3 and the biodegradable fraction content of 41.9±1.9% was placed in the reactor. The temperature of waste inside the reactor was measured by 9 Pt 1000 temperature sensors. The air for the process was constantly supplied from the outside of the reactor. Flow of the supplied air with temperature of 18.3±3.1°C was regulated depending on the average indication of all temperature sensors. Results of the temperature measurements were averaged and showed using Golden Software Surfer 7. As a result of the conducted research it was found that changes in the temperature inside the bioreactor occurred uniformly throughout its full volume. The time of reaching the temperature of 45°C (the beginning of thermophilic phase) was 25.6±4.0 hours (21 hours at the earliest). During the first period the temperature in the reactor was increasing most intensively in the lower parts of the layer, in the central part of the layer the temperature reached 45°C after 34 hours at the earliest, whereas on average it took 47.7±9.9 hours. The average maximum process temperature was 64.8±3.5°C.