Abstract: Mass spectrometry plays a crucial role in the structure elucidation of those effective substances from traditional Chinese medicine. Secure interpretation of the mass spectrum of each compound requires an adequate understanding of the key fragmentation process of the mass spectrum. In this paper, the regular patterns of mass spectrometry of lactones with unsaturated side chain and with saturated side chain from Ligusticum chuanxiong Hort. were reported. The diagnostic peaks at M⁺－29 and M⁺－42 show the molecular ion of the lactones losses a -C₂H₅ group and a -C₃H₆ group, respectively, the terminal alkyl groups from the unsaturated side chain. They are typical characteristic of the mass spectra of the lactones with unsaturated side chain from Ligusticum chuanxiong Hort., such as (Z)-ligustilide, (E)-ligustilide, butylidene phthalide and senkyunolide I. The characteristic peak at M⁺－57 displays the molecular ion of the lactones losses a -C₄H₉ group from the saturated side chain. This is typical characteristic of the mass spectra of the lactones with saturated side chain from Ligusticum chuanxiong Hort., such as butylphthalide and senkyunolide A.