The main components in illicit heroin -- O3-acetylmorphine, monoacetylcodeine, and O6-acetylmorphine -- were determined quantitatively; and the organic adulterants such as paracetamol, acetaminophen caffeine, and theophylline were detected qualitatively. Using these collected data, 500 seized illicit heroin samples were divided into 9 groups. The decomposition pattern of heroin was studied. The dependencies of both the decomposition patterns and the content ratios of monoacetylcodeine to heroin and monoacetylcodeine to O6-acetylmorphine on the source of the seized illicit heroin were observed. This information was used to develop a novel method for its source identification. An important factor for the component analysis of heroin samples by GC or GC/MS is the separation between heroin and other main components with similar molecule structure. As shown in the current experiment, under optimized conditions this method achieves their complete separation. The natural decomposition pattern of heroin products indicated a 3-month storage in air at room temperature does not affect the identification of heroin source based on the content ratios of monoacetylcodeine to heroin and monoacetylcodeine to O6-acetylmorphine. After storage of more than 3 months, the source identification based on the content ratio of monoacetylcodeine to the sum of O6-acetylmorphine and heroin is a more suitable method. 4 tables, 5 figures, and 23 references