Abstract: Litter size is a key parameter in understanding mammalian reproductive patterns.The amount of surviving litter size,the offspring size and the state of their mothers determine the reproductive fitness together.There are tradeoffs between the litter size and offspring size.While the offspring survival depends on the offspring size.However,the relationship between the litter size and offspring survival can still not draw a constant conclusion.The maternal fitness also is symbolized by the survival of young,so the litter size directly correlate with the maternal variables including the body mass and metabolic rate.There is the negative effect on the maternal survival,but the relationship between the litter size and maternal survival is unclear.In short,the study of litter size needs further explore,while fortunately recent advances in molecular techniques and the other means provide options for those as well.