Objective To investigate the effects of overfeeding during lactation on organ development of offspring mice. Methods A total of 20 Kunming pregnant mice were randomly divided into two groups,control and study group,which were treated with same condition during pregnancy.After delivery,all mothers reared their pups freely.At postnatal day 3,the litters of each group were culled to keep litter size at 4 pups/litter in study group and 10～12 pups/litter in control group.After weaning,all pups were fed with standard lab diet.Physical development of offspring was monitored from birth to postnatal 6 weeks.Organ coefficients of offspring major organs were determined and the morphology of them was observed through HE staining under microscope at age of 6 weeks. Results 1)Physical developmentat age of 3 and 6 weeks,the bodyweight of pups of study group were significantly higher than those of control group (P<0.05).2)Organ coefficientthe organ coefficients of heart,liver and spleen were significantly higher in offspring of study group than those in control group (P<0.05).3)Morphologyunder microscope,compared with offspring of control group,atrial and ventricular walls of heart were thicker,liver cells presented slight edema,and boundary of white pulp,red pulp and marginal zone in spleen was obscure in offspring of study group. Conclusions Overfeeding during lactation results in obesity during childhood and young adulthood,as well as morphological changes of some organs in offspring.These morphological abnormities might be the anatomical basis of influence of overfeeding during lactation on later disease.The profound mechanisms are worthy of further study.