Researches on sodium selenite (SS) mainly focus on production performance and rumen fermentation in ruminants, and the influence of dietary Se addition on ruminal microbial population and enzyme activity in dairy bulls is scarce. This study mainly evaluated the effects of SS on ruminal fermentation, microflora and urinary excretion of purine derivatives (PD) in dairy bulls. Eight ruminally cannulated dairy bulls were used in a replicated 4 × 4 Latin square design. Treatments were control, low SS (LSS), medium SS (MSS) and high SS (HSS) with 0, 0.1, 0.3 and 0.5 mg/kg of selenium (Se) from SS in dietary dry matter (DM), respectively. The supplement of SS (1.0 g/kg of Se) was mixed into the first third of the daily ration. Bulls were fed a total mixed ration with corn silage to concentrate ratio of 50:50 on a DM basis. Dry matter intake was not affected, average daily gain linearly increased, while feed conversion ratio quadratically decreased with increasing Se addition. The linearly increased digestibility of DM, organic matter, crude protein, ether extract, neutral detergent fibre and acid detergent fibre was observed. Both ruminal pH and ammonia-N concentration linearly decreased, whereas total volatile fatty acid concentration linearly increased. A lower acetate to propionate ratio was observed due to the unchanged acetate proportion and increased propionate proportion. Activity of cellobiase, xylanase, pectinase, α-amylase and protease, populations of total bacteria, fungi, protozoa, Ruminococcus (R.) albus, R. flavefaciens, Fibrobacter succinogenes, Butyrivibrio fibrisolvens and Ruminobacter amylophilus as well as urinary total PD excretion linearly increased, whereas populations of total methanogens and Prevotella ruminicola linearly decreased. The data indicated that dietary Se addition stimulated ruminal microbial growth and enzyme activity, and resulting in the increased nutrient digestion and growth performance, and the optimum supplementary dose of Se was 0.3 mg/kg dietary DM from SS in dairy bulls.