Lipid nanoparticles are prepared with solid fatty acids or esters, which is widely concerned in drug delivery due to their good biocompatiblity and safety. The oral bioavailability of poor-soluble drugs, peptides and proteins are very low because of their poor solubility, permeability or stability. However, lipid nanoparticles as their carrier can improve the oral bioavailability significantly, which enhance the application or investigation of lipid nanoparticles as oral drug delivery systems. This paper reviews the four aspects of lipid nanoparticles, including formualtion, preparation, absorption mechanism and application in oral delivery.