This experiment was aimed to study the effects of feeding acidified milk on growth performance and serum immune indexed of calves. A randomized comparative experiment design was adopted. Twenty healthy Holstein female calves at 3 days of age with the body weight of (40±5) kg were selected from a large-scale dairy farm, and were divided into 2 groups with 10 calves for each group. The control group fed pasteurized milk, the test group fed acidic milk, and both groups fed pellet diets ad libitum. The test period was 180 days (weaning at day 60). Feed intake of calves was recorded every day. Body weight of calves was measured at days 7, 30, 60, 90 and 180 of growth period, and daily weight gain was calculated. Meanwhile, blood was collected for measuring serum immunologic indexes. The results showed as follows:1) the milk urea nitrogen content of acidified milk group was significantly higher than that of pasteurized milk group (P<0.01), the number of Escherichia coli and total bacteria was significantly lower than that of pasteurized milk group (P<0.05), and there was no significant difference in lactose percentage, milk fat percentage and milk protein percentage between groups (P>0.05). 2) At days 31 to 50, the liquid feed average daily feed intake of acidified milk group was significantly higher than that of pasteurized milk group (P<0.01); at days 61 to 90, the solid feed average daily feed intake of acidified milk group was significantly higher than that of pasteurized milk group (P<0.01). 3) At days 60 to 180, the average daily gain of acidified milk group was significantly increased compared with pasteurized milk group (P<0.05). At day 60, the body straight length, body height and cannon bone circumference of acidified milk group were significantly higher than those of pasteurized milk group (P<0.05); at day 90, body straight length, body oblique length, body height and cannon bone circumference were significantly higher than those of pasteurized milk group (P<0.05); at day 180, the body straight length, body oblique length and body height of acidified milk group were significantly increased compared with pasteurized milk group (P<0.01). 4) At day 60, serum immunoglobulin (Ig) A content of acidified milk group was significantly higher than that of pasteurized milk group (P<0.05); at day 90, serum IgG content of acidified milk group was significantly higher than that of pasteurized milk group (P<0.01); at days 90 and 180, serum IgM content of acidified milk group was significantly higher than that of pasteurized milk group (P<0.05). At day 30, the contents of serum tumor necrosis factor-α (TNF-α) and interleukin (IL)-1β of pasteurized milk group were significantly higher than those of acidified milk group (P<0.01), and the content of serum IL-6 was significantly higher than that of acidified milk group (P<0.05); at day 60, the content of serum IL-1β of pasteurized milk group was significantly higher than that of acidified milk group (P<0.01), and the content of serum IL-6 was significantly higher than that of acidified milk group (P<0.05); at day 90, the content of IL-1β of pasteurized milk group was significantly higher than that of acidified milk group (P<0.05). In summary, acidification of formic acid milk has little effect on the composition of milk, can inhibit the growth of harmful bacteria in milk, improve the palatability, feed intake and immunity.