Abstract The current study aims to evaluate the effects of different gestation dietary Met/Lys (methionine, Met/lysine) ratios on the production performance of sows. Specifically, it measured the effect of Met on plasma urea and AA concentrations and placental vascular density of pregnant sows. A total of 325 multiparous sows (third parity, Large × White) were randomly allocated to five dietary treatments (n = 65) with five dietary Met/Lys ratios 0.27 (nutrient requirements of swine NRC 2012 level), 0.32, 0.37, 0.42, and 0.47). The litter size and weight at birth were measured and recorded. Blood samples were obtained on days 0, 40, 90, and 114 of gestation, and placenta samples were collected at parturition. The effects of different dietary Met/Lys ratios on the reproductive performance were evaluated based on the prolificacy of sows as either high (≥13 total piglets born) or low (<13 total piglets born). The results showed that dietary Met/Lys ratio had no significant effect on the reproductive performance of lower prolificacy sows (P > 0.05). However, for high-prolificacy sows, litter weight of born alive significantly increased in 0.37 Met/Lys ratios group compared with control group (P < 0.05). The gestation dietary Met/Lys ratio showed significant quadratic effects on the litter birth weight and percentage of piglets born with weight <0.9 kg (P < 0.05), and the Met/Lys ratios to achieve the best reproductive performance determined to be 0.37. Furthermore, plasma urea concentrations of sows also changed with Met/Lys ratios quadratically (P < 0.05). Increasing dietary Met/Lys ratios elevated the concentration of most plasma AA. Although the dietary Met/Lys ratio had no significant effect on the placental vascular density (P > 0.05), the gestation dietary Met/Lys ratio showed significant quadratic effects on the placental vascular density (P < 0.05). In addition, the birth weight of piglets of high-prolificacy sows was positively correlated with the placental vascular density (P < 0.01). Taken as a whole, the dietary Met/Lys ratio showed a quadratic curve relation with birth weight performance and placental angiogenesis performance, to which 0.37 ratio contributed to the best performance of high-prolificacy sows.