Abstract: Based on the Lighthill equation and the Green's theory, the acoustic radiation mechanisms of the laminar to turbulent transition region are discussed and analyzed, and the fundamental types of sound sources at transition region are classified. It is shown that the acoustic field are generated by a fully developed turbulent flow with a modulation due to the intermittency and the two additional sound sources; the fundamental types of sound sources at transition region include monopole, dipole, and quadrupole source distributions; and Lauchle's phenomenological model for predicting the acoustic radiation of transition region is proved to be correct and reliable.