Abstract: Withthe help of traditional paraffin-section method, the structure, distribution, initiationand development of resin ducts of the primary structure of stem in Pinus massoniana wereinvestigated. The results were as follows the resin ducts were found in both cortex andprimary xylem. At the level 390 micrometer from the shoot tip, a group of the corticalduct initials developed from the ground meristematic cells which are located at both sidesof the procambium bundle. The development can be divided into 5 stages the initial stage,the intercellular layer swelling stage, the intercellular space forming stage, the ductexpanding stage and the mature stage. In the primary xylem, the duct initial cellsdeveloped from the procambium cells. The style of development was similar to that of theresin ducts of cortex. The difference of structure between two kinds of resin ducts instem primary structure and the reason for delaying initiation of cortical resin ducts inP. massoniana, compared with P. tabulaeformis, were analyzed and discussed.