Abstract: Waste milk is unfit for human consumption because it might be harmful for human health. However, it may be acceptable for calf feeding if properly processed. This study was designed to investigate the effect of 2 different sterilization methods of waste milk on diarrhea occurrence of calves. Forty female Holstein calves were randomly assigned into one of 2 groups and fed either acidified (by formic acid) or pasteurized waste milk and defined as treatment group (TT) and control group (CT). The total number of bacteria in milk was detected and found higher in pasteurized milk than in acidified milk (3.6 × 10⁴ vs. 3.8 × 10³ cfu/mL) on average. The birth weights of calves in 2 groups were similar (TT: 43.38 ± 4.100; CT: 43.83 ± 4.854; P > 0.05). All calves were fed 4 L of colostrum within 1 h after birth and were subsequently fed milk 3 times per day at 0730, 1430, and 1930. Fecal samples were collected from all calves daily for 8 weeks from the third day after birth. Feces were scored by UW-Madison calf health scoring. Calf diarrhea occurrence was described by 3 indexes including diarrhea rate (DA), diarrhea frequency (DF) and fecal indexes (FI). The calculation equations are as bellow. Data were statistically analyzed using the Two-Sample t-test for means procedure of SAS (version 9.2, SAS Institute Inc., Cary, NC). Diarrhea rate = number of diarrhea calves/total number of calves × 100%; diarrhea frequency= (∑ number of diarrhea calves × days in diarrhea)/total number of calves × number of days on trial × 100%; fecal index = the sum of fecal score/(total number of calves). Results indicate that the overall calf diarrhea in TT group was lower compared with CT group. DF and FI of TT group were lower than those of CT group (DF: 10.00 ± 0.02 vs. 15.9 ± 0.02%; FI: 1.41 ± 0.28 vs. 1.59 ± 0.25; P < 0.05). DR values were not significantly different between two groups during the whole trial period of 8 weeks (P > 0.05); however, DA, DF and FI within the first 6 weeks (d3 to d42) was lower in TT group than those in CT group (DR: 60 vs. 90%; DF: 3.75 ± 1.07 vs. 7.50 ± 1.10%; FI: 1.34 ± 0.029 vs. 1.50 ± 0.03; P < 0.05). We conclude that feeding waste milk acidified with formic acid can effectively reduce calf diarrhea occurrence compared with feeding pasteurized waste milk.