Chemical composition and it`s influences on the potentially digestible fractions and fermentation rates of alfalfa and orchardgrass were estimated in in vitro method. Alfalfa and orchardgrass were harvested at early bloom and late vegetative stages, respectively. Alfalfa had lower cell wall constituents (CWC) and higher lignin content than orchardgrass, and lignin and cellulose ratios were 0.26 in alfalfa and 0.13 in orchardgrass. Potentially digestible dry matter, cell wall constituents, acid detergent fiber (ADF) and cellulose were 57.3%, 37.4%, 41.6% and 63.59% in alfalfa and 73.1%, 59.9%, 61.5% and 86.6%, respectively in orchardgrass. Fermentation rates per hour (0-72 hour) of dry matter, CWC, ADF and cellulose were 0.66%, 0.70%, 0.59% and 1.17% in alfalfa and 1.21%, 0.90%, 1.00% and 2.21 % in orchardgrass, respectively, which were much higher in orchardgrass, but the potentially digestible fractions were ranged from 4.32% to 5.30% in alfalfa and from 3.21% to 4.39% in orchardgrass, which were higher in alfalfa.