This study aimed to explore the correlation between the calf birth weight and leptin or insulin level in venous serum of the puerperal cow, calves or cords. We chose 54 healthy Holstein cows that experienced normal natural birth, divided them into group A (calf birth weight ≤ 40 kg, n = 9), group B (calf birth weight 40﹣45 kg, n = 25) and group C (calf birth weight ≥ 45 kg, n = 20), and collected the venous serum samples from the puerperal cow, the calf and the umbilical cord, respectively. The expression levels of serum leptin and insulin were detected by ELISA. Single factor analysis of variance and bivariate Pearson were used to analyze the correlation of the expression levels of leptin and insulin in these three kinds of venous sera, as well as their correlation with the calf birth weight. The results demonstrated that: (1) Leptin and insulin in venous serum of puerperal cows were significantly higher than in calves and cords (P < 0.01), while leptin and insulin in venous serum of puerperal cows had no significant correlation with that of calves or cords (P > 0.05) (Table 1). (2) There was no significant difference in the expression level between leptin and insulin in venous serum of calves and that of cords (P > 0.05), while leptin and insulin in venous serum of calves had highly significant positive correlation with that of cords (P < 0.01) (Table 2). (3) There was significant positive correlation between leptin and insulin in venous serum from both puerperal cows and cords (P < 0.05), but there was no significant correlation in calves (P > 0.05) (Table 3). (4) The calf birth weight had no significant correlation with leptin and insulin in venous serum of puerperal cows (P > 0.05); however, it had significant positive correlation with leptin in venous serum (P < 0.05) and highly significant positive correlation with insulin in venous serum (P < 0.01) from calves and cords (Table 4). (5) There was no significant difference in Leptin and insulin levels in venous serum from puerperal cows, calves or cords between male and female calves (P > 0.05) (Table 5). In conclusion, leptin and insulin expression levels in puerperal cows were significantly higher than those of calves and cords. The calf birth weight had no significant correlation with leptin and insulin in venous serum of puerperal cows, while it had significant positive correlation with leptin and insulin in venous serum from both calves and cords.