This experiment was conducted to study the effects of dietary selenium yeast on the growth performance, blood routine and serum biochemical indexes of Dorper×thin-tailed Han crossbred sheep. Forty Dorper×thin-tailed Han crossbred weaning lambs with the similar body weight (about 34 kg) and good health were randomly assigned to 4 groups with 10 sheep in each group, and all sheep were housed in individual pen. Sheep in the 4 groups were fed experimental diets supplemented with 0 (control), 0.2, 0.4 and 0.8 mg/kg selenium (the form was selenium yeast) based on a basal diet, and the measured values of the selenium content in the experimental diets were 0.074 (control), 0.307, 0.491 and 0.846 mg/kg, respectively. The experiment lasted for 10 days for adaption, and 20 days for test. The results showed as follows:average daily gain (ADG) was firstly increased and then decreased with selenium supplemental level increasing, and the highest value of ADG was found when selenium supplemental level was 0.2 mg/kg. The lowest value of feed/gain (F/G) was found when selenium supplemental level was 0.2 mg/kg, and it was significantly lower than that in the other three groups (P<0.01). The highest platelet count was found when selenium supplemental level was 0.8 mg/kg, and it had significant difference compared with the other three groups (P<0.01). The highest basophilic granulocytes (BASO) percentage was found when selenium supplemental level was 0.8 mg/kg, and it had significant difference compared with the 0.4 mg/kg group (P<0.01). Dietary supplemented with different levels of selenium had no significant effects on other blood routine indexes (P>0.05). The highest values of serum total protein (TP), urea nitrogen (UN) and creatinine (CREA) contents were found when selenium supplemental level was 0.8 mg/kg, and they had significant differences compared with the other groups (P<0.05 or P<0.01). The activities of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in 0.8 mg/kg group were the highest, which were significantly higher than those in the control group (P<0.05 or P<0.01). The serum alkaline phosphatase activity in control group had the highest value, and it had no significant difference compared with 0.2 mg/kg group (P>0.05). The highest value of serum γ-glutamyltransferase (γ-GGT) activity was found when selenium supplemental level was 0.8 mg/kg, and it had significant difference compared with the other groups (P<0.01). Phosphonium content in serum was the highest when selenium supplemental level was 0.8 mg/kg, and it had significant difference compared with 0.2 and 0.4 mg/kg groups (P<0.01). Iron content in serum was the lowest when selenium supplemental level was 0.8 mg/kg, which was significantly lower than that in control group (P<0.01), and it in 0.4 mg/kg group was also significantly lower than that in control group (P<0.01). Considering all indexes of this experiment, the appropriate selenium (the form is selenium yeast) supplemental level is 0.2 mg/kg in the diet of Dorper×thin-tailed Han crossbred sheep, and the measured value of selenium content in the diet is 0.307 mg/kg.