Due to the absence of detailed kinetic parameters,one of the most important methods,network topology based metabolic pathway analysis has been developed.The paper firstly introduces the brief history of network based on pathway analysis.Then,two such network based methods are emphasized:Elementary modes and extreme pathways,with their similarities,differences and applications.At last,extreme pathways are employed to analyze PHB metabolism of B.thuringiensis.Except forhistoricalbiochemical pathways,the results also suggest that there are some novel pathways.