In this paper, the combustion noise of a compression ignition (CI) diesel engine operating with biodiesels has been investigated experimentally. It aims to explore an effective method for combustion process monitoring and fuel quality evaluation through analysing the characteristics of the engine combustion noise. The experiments were conducted on a four-cylinder, four-stroke, direct injection and turbocharged diesel engine fuelled with biodiesels (B50 and B100) and normal pure diesel, and operating under different loads and speeds. The signals of cylinder head vibration, engine noise and in-cylinder pressure were measured during the tests. A coherent power spectrum analysis method was used to investigate the vibration and noise signals that related to the combustion process. The results shown that the noise components at the frequency band of 2 -3 kHz are closely related to the combustion process. Subsequently, the Wigner-Ville distribution is employed to present the energy distribution of engine noise in the time-frequency domain. Then a band-pass filter based on fractional Fourier transform (FRFT) is developed to extract the main component of the combustion noise for feature extraction. The results show that the sound pressure levels (SPLs) of the extracted combustion noise of the test diesel engine fuelled with biodiesels are higher than that fuelled with diesel. This is also identical to the variation of in-cylinder pressure. The results demonstrate that the features of the extracted combustion noise can indicate the combustion characteristics and provide useful information for monitoring the combustion process and evaluating the fuel quality of diesel engines.