Although magnesium (Mg) contents are different between breast milk and formula, few studies have investigated the blood Mg level in breast fed or formula fed preterm infants. We examined the influence of feeding type on serum Mg and whole blood ionized Mg (iMg) levels in preterm infants soon after birth. We included 115 preterm infants born between gestational weeks 32 and 35. Infants were separated into two groups: breast milk (BM) dominant group (n = 30) receiving ≥70% of Mg intake from BM and mixed-fed (MF) group (n = 85) receiving ≥30% of Mg intake from formula. Blood levels of Mg, iMg, Ca, and iCa at day 1 of age and at discharge from the hospital were compared between the groups. No differences in the Mg and iMg levels at day 1 of age were observed between the two groups. The Mg and iMg levels at discharge were significantly lower (P < 0.05) in the BM group than in the MF group; 0.86 (interquartile range 0.81-0.91) versus 0.91 (0.86-0.99) mmol/L and 0.46 (0.41-0.51) versus 0.52 (0.47-0.57) mmol/L, respectively. There were no differences in the Ca and iCa levels between the two groups. By stepwise multiple regression analysis, the percentage of BM intake was a significant independent predictor of the Mg and iMg levels. The feeding type influenced serum Mg and blood iMg levels in preterm infants soon after birth. Further studies are needed to investigate the influence of Mg on growth and the optimal range of blood Mg levels.