This study was designed to determine effect of various plant protein sources in high-quality feed block (HQFB) on feed intake, rumen fermentation, and microbial population in swamp buffalo. Four rumen-fistulated swamp buffaloes (Bubalus bubalis) were randomly assigned according to a 4 × 4 Latin square design. Four kinds of plant protein sources (coarse rice bran (CRB), cassava hay (CH), Phaseolus calcaratus hay, and mulberry hay (MH)) were mixed in the HQFB. HQFBs were allowed to be licked at free choice, and urea-lime-treated rice straw (ULRS) were fed ad libitum. It was found that bacterial population and fungal zoospores in CH-fed group tended to be higher than those in other groups. Moreover, protozoal population in CH, P. calcaratus hay, and MH were lower than those in CRB supplemented group (P < 0.05). Cellulolytic bacterial population was highest in CH-fed group while proteolytic bacteria population was highest in P. calcaratus hay-fed group (P < 0.05). CH-fed group had higher ULRS intake than those in other groups (P < 0.05). Nutrient digestibility of CP, NDF, and ADF in CH-fed group was significantly higher than those in other groups (P < 0.05). Total VFA was highest in CH-fed group (P < 0.05). N absorption was highest in CH-fed group (P < 0.05). Based on this study, it could be concluded that cassava hay, P. calcaratus hay, and mulberry hay are potential to be used as protein sources in the HQFBs especially cassava hay.