Factors which influence on the changes of chemical composition and digestibility with the advance of growth stage of a grass were investigated with special attention to characteristic pattern of formation and development of tillers consisting an individual plant. With two tropical grasses, dallisgrass(Dg) and buffelgrass(Bg), the relations were studied between the changes of crude protein, cellulose, lignin and in vitro dry matter digestibility(IVDMD), and the changes of the ratio of unculmed(UT) to culmed tillers(CT). The experiment was conducted from boot stage to wilting stage of the main shoot. Obtained results are summarized as follows; 1.The total number of tillers was not increased during the experimental period in both of the grasses. As to Bg, internodal elongation of tillers occurred almost simultaneously and a half of the tillers had culmed by heading stage of the main shoot. Tillers of Dg, on the other hand, started to elongate successively and a half of them had culmed by wilting stage of the main shoot. 2.Dry matter weight percentage of UT dropped from 70% to 15% in Dg and from 16% to less than 2% in Bg. Weight percentage of stems of CT showed rapid increases in both of the grasses. 3.Crude protein content and IVDMD were lower and cellulose and lignin contents were higher in CT than UT in both of the species. In each fraction and whole plant of respective species, crude protein and IVDMD showed decreases and cellulose and lignin showed increases with the advance of growth. Their changing rates were more rapid in Bg than Dg. 4.The changes of the chemical component contents and IVDMD of each fraction especially of stems of CT which occupied larger parts of total dry weight, exerted major influences on chemical composition and IVDMD of whole plant comparing to the changes of dry matter weight ratio among component fractions. 5.One of the factors which produced the differences between the species in the changing rates of chemical composition and IVDMD with growth supposed to be resulted from the difference of tillering habits or patterns of their formation and development.