Abstract: Root characters of four alfalfa varieties in greenhouse were compared. WL323 alfalfa had the longest main root （48.46 cm）. American Alfalfa King had longer diameter of main root, more secondary roots as well as larger root biomass and root volume than other three varieties. Any two indexes of root （diameter of main root, length of the thickest secondary root, diameter of the thickest secondary root, number of secondary roots, root biomass and root volume） except length of main root were positively correlated （significantly or extremely significantly）.