An MCU based programmable frequency detector was developed and relative tests were carried out. The detector used few elements which are very common and cheap, one MCU which has only 8 pins and with small dimension, one Schimidt trigger, one D type flip-flop and several resistors and capacitors. Cost for the detector is very low. Besides hardware work, software program has played very important role in the detector. By adjusting program parameters, different frequency can be detected and detecting band width can also be adjusted. Program flow chart is given in this thesis. Test shows that the detector is stable and sensitive and is suitable for different applications, e.g. over power line data transmission, etc. This paper introduces theory of the detector, hardware design and the software programming.