Abstract: In many modern aircraft, flap side-edge noise is a very important noise, in some case the most intense, source of airframe. A large planar microphone array has been successfully applied to measure flap side-edge noise for landing commercial aircraft. In order to understand the mechanism of the flap side-edge noise, to obtain the detailed and reliable noise prediction schemes and suppression methodology for this source noise emission, a theoretical analysis of the flap side-edge noise is presented in this paper. It is found that the broadband noise features of flap side-edge noise source can be predicted well by the Howe's theory.