To determine the effect of Acacia pods meal (APM) on the rumen bacterial diversity, three fistulated cattle were fed at 3% of BW with Napier, Napier-rice bran-copra meal mix (RBC) and Napier-APM following a 3 x 3 Latin square design. Isolated 16S rDNA from the rumen were sequenced and analyzed in silico. Growth performance of cattle was also evaluated using 18 heifers divided into three groups and fed with 70% Napier grass and 30% of the following treatments as concentrate portion: 30% RBC, 15% RBC:15% APM and 30% APM. Secondary metabolites extracted from APM were qualitatively assayed by paper chromatography (PC) and thin-layer chromatography (TLC). Tannin content was determined spectrophotometrically. Bacterial diversity analysis showed that feeding APM resulted in the proliferation of tannin-resistant bacterium S. ruminantium and loss of cellulolytic bacteria A. ruminis. Feeding APM in growing heifers reduced average daily gain and feed conversion ratio but not average daily feed intake. Hot water extract contained hydrolyzable tannins as determined by PC. TLC of n-hexane extract did not show secondary compounds. Quantitative analysis showed 16.42% tannins in APM. The study showed that reduction in growth performance of heifers is related to tannins in APM that could have direct bioactivity against important bacteria in the rumen.