The ruminal parameters, in vivo, in vitro, in sacco digestibility, and water extractable dry matter (DM), were measured in buffaloes fed rice straw and elephant  grass  with  supplementation of fresh leaves of Sesbania grandiflora. The design was a 2x2 factorial arrangement with three replications. The first factor of the experiment was feed (rice straw or elephant grass); the second factor was supplementation of 4 kg/day of fresh leaves of Sesbania grandiflora. The buffaloes received the forage at 80% of their requirement and the sesbania leaves were supplemented once in the morning. Ruminal ammonia concentration, bacteria population and volatile fatty acids were significantly higher for the diets with Sesbania grandiflora supplementation. Supplementation also significantly improved in  vivo  DM  and  NDF digestibility but had no effect on in vitro measurement of digestibility nor on “a”, “b” and “c” parameters in the in sacco test. However, the effective DM degradability value as measured by the in sacco method was significantly increased by supplementation compared to the control. In vivo DM digestibility was more closely correlated with water extractable DM values than with any of the in vitro analyses. It is concluded that farmers could supply a supplement of Sesbania grandiflora leaves to  buffaloes  fed  low  quality  forages  to improve their rumen function and production.