Elephantgrass(Pennisetum purpureum)is an integral component as perennial herbaceous plant in tropical and subtropical regions.Its advantages include high yield,strong regeneration ability,excellent resistance,high nutritional value and the wide range of applications,etc.In this study,five elephantgrass cultivars,sampled in March,May,July,August,October and December in 2008,were tested to determine the dynamic changes of enzyme activities of phenylananine ammonia-lyase(PAL) and 4-coumarate:CoA ligase(4CL) in stems.Results indicate that the PAL maximum activities of four cultivars('MT-1','N51','Mott' and 'Guimu No.1') appeared in July,while 'Huanan' peaked in May.The maximums were consistent with their fast increasing stages of stem lignin content.PAL minimum activities were in different months for different cultivars,showing the differentiations among these five cultivars.The maximum activities of 4CL for the five cultivars were in May,and sharply decreased in July,but with a little activity peak values appeared in October for 'MT-1','N51','Huanan' and 'Guimu No.1'.'Mott',had no peak value in October,but a non-significant peak value in December due to the delaying maturity stage.The maximums of 4CL appeared generally before the fast lignin content incre,which also was the prophase of fast jointing stage.These research findings can provide important helps for the breeding and extension of 'MT-1' new cultivar.