This paper presents a method to identify the radiation noise source of diesel engine based on a Cepstrum Analysis (CA) and Analytic Hierarchy Process (AHP). Through measuring noise and surface vibrations of a diesel engine, cepstrum analysis is applied to the measured signals. In an investigation of the radiation noise source, the hierarchy tree and judgment matrix are given. Through identification, the main noise source is found. The results show that the left body contributes maximally to non-harmonic spectral characteristics of noise. Therefore, in order to reduce engine vibration and noise effectively, we should optimize and improve left body. In addition, optimizing and improving gear chamber casing, oil pan, inlet pan and valve chamber cover can decrease harmonic vibration and noise of engine.