The density functional theory is used to calculate the vibration frequency of biodiesel and diesel, identify the vibration modes and demonstrate the correlation between terahertz spectra and cetane number. The reason that biodiesel shows higher terahertz absorption and cetane number than diesel can be comprehended to be the influence of ester group at the end of the C-chain. The terahertz absorption and centane number of biodiesel-diesel blends increase with the increase of volume percentage of biodiesel in mixture, and a congruent relationship is established between absorption curve and centane number. As a result, the cetane number of biodiesel-diesel blend will be confirmed by its terahertz spectra.