Abstract: Elephant grass (Pennisetum purpureum), a valuable forage in south China, is a typical clonal plant. Modular theory was used to analyze the characteristics and dynamics of modular biomass and its allocation ratio in five elephant grass cultivars (MT-1, Mott, Huanan, N51 and Guimu No.1) from 2007 to 2009, with six harvests. The allocation ratios of culm and leaf blade were the highest and second highest respectively of the modules which contributed to the accumulation of photosynthate in the vegetative growth period. From vegetative growth to reproductive growth, the leaf biomass diminished but dead leaf biomass and culm biomass increased. Similarly, the biomass of three internodes of the culm base and its allocation ratio reduced while the culm biomass and its allocation ratio increased.