Abstract:This paper provides a summary of the role of plant hormone in vascular differetiation.Auxin is a major factor in regulating cell differetiation of the xylem, cytokinin plays an important role of bringing meristematic initials and cells differetiation, gibberellin promotes fiber formation,an application of combining of both auxin and gibberellin can stimulate fiber differetiation of secondary xylem,ethylene promots the differetiation of xylem and phloem in tree.This paper discusses three major problems during wood formation.First,the level of auxin controls size and density of vessel and tracheid in xylem along the tree axis.Second,environmental factors influence cells differetiation of xylem.Third,regard with to the machine of ring-porous versus diffuseporous wood formation in temperate deciduous hardwood trees.A new hypothesis was put forwarded regarding the growth regulator control of wide earlywood vessel formation in ring-porous trees and the control of narrow vessels and fibers in the latewood. This discussion is of great value in terms of improving wood productivity and quality in the future.