We investigated the differences in serum biochemistry levels between breast-fed and formula-fed infants. We provide reference values of serum biochemistry levels for 4-week-old (w/o) and 8 w/o infants for future clinical applications.Sixty healthy infants were enrolled in this study (30 infants were included in the breast-fed and formula-fed groups, respectively). During hospital visits at 4 and 8 w/o, several parameters, including body weight, body length, and head circumference were recorded. Blood was drawn to measure 14 serum biochemistry markers.There were no significant differences between the two groups in terms of growth or anthropometric measurements. Serum cholesterol, triglyceride (TG), alanine aminotransferase (ALT), aspartataminotransferase (AST), gamma glutamyl transferase (GGT), total bilirubin (T-bil) and direct bilirubin (D-bil) levels were significantly higher in the breast-fed group compared with those measured in the formula-fed group at both 4 and 8 w/o. Serum blood urea nitrogen (BUN) and inorganic phosphate (IP) levels were significantly lower in the breast-fed group compared with the formula-fed group at 4 and 8 w/o. In the formula-fed group, serum total protein (TP), albumin, cholesterol, uric acid (UA), ALT, and AST levels were significantly higher in 8 w/o infants compared with 4 w/o infants; in addition, serum GGT, T-bil, D-bil, IP, and iron (Fe) levels were significantly lower in 8 w/o infants compared with 4 w/o infants. In the breast-fed group, serum TP, albumin, and UA levels were significantly higher in 8 w/o infants compared with 4 w/o infants; in addition, serum BUN, GGT, T-bil, D-bil, IP, and Fe levels were significantly lower in 8 w/o infants compared with 4 w/o infants.Different sources of nutrition may result in different metabolic responses; these differences are reflected by different serum biochemistries. The reference values for serum biochemistry levels also differ according to the infant's postnatal age.