This experiment was conducted to investigate the effects of dietary betaine (BT) and coated betaine (CBT) on growth performance, nutrient digestion and rumen fermentation of Holstein bulls. Thirty-six Holstein bulls of (238.45±0.89) day-old with an average body weight of (304.0±7.4) kg were randomly divided into 3 groups with 12 bulls per group. Bulls in the control group were fed a basal diet, and the others in BT group and CBT group were fed the basal diet supplemented with 6 g / d BT and 24 g / d CBT, respectively. The pre-trial period lasted for 20 days and the experimental period lasted for 60 days. The results showed as follows: 1) compared with the control group, the average daily gain (ADG) of bulls in BT and CBT groups was significantly increased (P<0.05), and the feed conversion rate (FCR) was significantly decreased (P<0.05). The ADG of bulls in CBT group was significantly higher than that in BT group (P<0.05). 2) Compared with the control group, the apparent digestibilities of dry matter (DM), organic matter (OM) and neutral detergent fiber (NDF) of bulls in BT and CBT groups were significantly increased (P<0.05). The apparent digestibilities of DM and OM of bulls in CBT group were significantly higher than those in BT group (P<0.05). 3) Compared with the control group, the acetate molar percentage and acetate to propionate ratio in rumen of bulls in BT and CBT groups were significantly increased (P<0.05), while the propionate molar percentage in rumen was significantly decreased (P<0.05); the numbers of total bacteria, total anaerobic fungi, total protozoa, Ruminococcus albus, Ruminococcus flavefaciens, Butyrivibrio fibrisolvens, Fibrobacter succinogenes and Ruminobacter amylophilus in rumen in BT and CBT groups were significantly increased (P<0.05). The numbers of total anaerobic fungi, total protozoa, Ruminococcus flavefaciens, Butyrivibrio fibrisolvens and Ruminobacter amylophilus in rumen of bulls in BT group were significantly higher than those in CBT group (P< 0.05). 4) Compared with the control group, the contents of total protein (TP), albumin (ALB), growth hormone (GH) and insulin-like growth factor-Ⅰ (IGF-Ⅰ) in serum of bulls in BT and CBT groups were significantly increased (P<0.05). The ALB content in serum of bulls in CBT group was significantly higher than that in BT group (P<0.05). In conclusion, dietary BT and CBT can improve growth performance, nutrient digestion and rumen fermentation of Holstein bulls, and CBT has better effects on ADG and apparent digestibilities of DM and OM than BT.