The aim of this study was to compare the in vitro true digestibility (IVTD) and in vitro NDF digestibility (IVNDFD) of some leguminous hays (field pea, common vetch, alfalfa) and graminaceous straws (wheat and rice) in combination with the rumen fluids collected from water buffaloes or Holstein cows as inoculum. Each filter bag, which contained one of the five feed blends, buffer solution and rumen fluid, was placed in a Daisy' incubator digestion jar. Each feed treatment was replicated 8 times and the incubation period was 48 hours. There were significant differences between the effects of the rumen fluids of the water buffaloes and Holstein cows on some fermentation characteristics of different substrates. The mean TVFA and NH3-N concentrations for both ruminant species were significantly higher for leguminous hays than graminaceous straws. These concentrations were not significantly different between rumen fluid applications of water buffalo and Holstein cow for leguminous hays, and the same was true for wheat straw. The rumen fluid of the water buffaloes increased the IVTD and IVNDFD values of the low quality feeds (wheat and rice straw) in comparison to the rumen fluid of Holstein cows. This study also demonstrated that leguminous hays have higher nutritional quality than graminaceous straws due to their higher nitrogen and lower fibre contents. Additionally, the results of this experiment suggest that the water buffalo rumen fluid generated higher microbial biomass production when combined with the graminaceous straws than with the rumen fluids of Holstein cows, which was reflected in a higher level of synthesis of VFAs.