Abstract: Accurate recognition of sow oestrus is the key technique for identifying the optimal mating time and improving insemination. But on pig farms it is most often done by the observation of workers. In this paper a microprocessor controlling infra-red detecting system were successfully developed and used to recognize the oestrus of sow. This system comprised infra-red sensor and detecting circuit, microprocessor sampling and storage circuit for infra-red signal and other electronic circuits. The working principle of this system for recognizing was also illustrated. Case study results of the system on a pig farm in Belgium showed that the recognition validation for sow oestrus was above 80%, and the system could be used to reduce effectively the non-productive days of sow and the farm labor cost.