The longitudinal study was done to study the effect of length of lactation on mineral content of breast milk and to find out basic data on nutritional status for infants and lactation women in Korea. Milk was collected from primiparae(n=10) and multiparae(n=6) at 0.5, 1, 2, 3, 4 and 5 months postpartum. During lactation the content of total ash, calcium, phosphours decreased significantly (;<0.001), but magnesium increased significantly (p<0.001). Total ash, calcium, phosphrus and magnesium contents of breast milk per 100ml were as 0.22g, 27.25mg, 14.67mg and 2.77mg in total lactating women(n=16), respectively. These contents were not significantly different between primiparae and multiparae. A ratio of calcium to phosphorus was about 1.88:1 in breast milk and 0.50:1 in maternal dietary intake. No correlation was found between maternal dietary intake and mineral content of breast milk.