Abstract: Three High Mountain Finewool wethers (one and a half years old and average live weight of 24 kg) fitted with permanent rumen fistula were used to study the effects of tannin content in Sainfoin（Onobrychis viciifolia） hay on apparent digestibility of nutrients and utilization of nitrogen (N) in sheep diets. With a 3×3 Latin square design, the experiment was divided into three periods, each phase of 19 days, the whole experiment lasted 57 days. The tannin contents were 0.00 g·kg^-1 dry matter (DM) of diet A (the control), 1.70 g·kg^-1 DM of diet B, and 3.40 g·kg^-1 DM of diet C, respectively. Correspondingly, the condensed tannin contents were 0.00 g·kg^-1 DM of diet A, 1.52 g·kg^-1 DM of diet B, and 3.03 g·kg^-1 DM of diet C. The results showed that the tannin contents in Sainfoin hay among three diets have no significant (P＞0.05) effect on the digestibility of DM, organic matter (OM), phosphorus (P), neutral detergent fiber (NDF) and acid detergent fiber (ADF) of diets, but there was a decreasing tendency towards the digestibility of calcium (Ca) （P=0.087）. Compared with diet A and B, diet C showed the better effect on protein protecting, the significantly lower(P＜0.05) excretion of urinary nitrogen and the significantly higher(P＜0.01) retention of N. Therefore, under this experiment condition, the preferable effect on the protein protection was found at 3.40 g·kg^-1 DM tannin (3.03 g·kg^-1 DM condensed tannin) of diet.