Abstract: Six loblolly pine trees of 27 a were cut down as analysis trees and sliced into 3 ring group wood samples for the determination of wood basic density,the within-tree variations of basic density were studied in details.The results showed there existed radial variation and vertical variation of the ring wood basic density and the cross section wood basic density of certain number of rings from pith.The radial variation from pith to bark at any given height is that the basic density increases from pith outward for about the first 15 rings and then levels out,the vertical variation from trunk base to top is that the basic density is the highest near the base and decreases fast upward within 3～5 m butt and then stays essentially constant with increasing height.The regression analyses show the certain ring wood basic density and cross section wood basic density is the function of ring position from pith and sample height, BD=a+b/R+c/H .The age of transition from juvenile to mature wood is about 12 a,the wood basic density correlation of young timber stage and mature timber stage become notable and steady with the increase of age of young timber,the early selection for wood basic density would be done after the transition age of juvenile to mature wood.