Melatonin (MEL) is associated with the formation of incremental lines and calcification of teeth dentin. However, the effects of MEL intake during osteogenesis on bone formation in rats have not been clarified. This study aims to investigate the effect of MEL intake in lactating rats on the morphology and quality of lactating bone. The lactating rats were divided into three groups: 1 control group (Con), 2 low-concentration group (Low), and 3 high-concentration group (High). HaMT (6-hydroxymelatonin) value increased in proportion to the MEL dose. MEL intake in the pregnant rats was transferred to child rats through breast milk. In the MEL-treated group, the trabecular area increased. The bone mass is considered to be promoted by MEL administration. However, the bone in the MEL-treated group was considered immature. The effects of the MEL mechanism on both calcification and formation of bone may differ from those of dentin.