Abstract:In order to disdnguish the juvenile wood from mature wood in plantation or natural forest of China fir (Cunninghamia lanceolata) and Japanese larch (Larix kaempferi), a variation distribution of the averge tracheid feature value was applied to the logatithmical fonmula y = a +blnx, where y was the tracheid feature value (tracheid length, tracheid width and fibril angle), x was the ring number or dislance from the pith, and the effect of growth rate on the increment of juvenile wood were Studied. The results show that the regression curves oblanied from this logarithmical formula agreed well with the actual variation and the coefficient of correlation R between tracheid length and ring numher or distance from the pith (0.98), so the ratio among tracheid length, tracheid width and fibril angle was suitable to he used to differentiate the juvenile wood zone. Furhemore, the increasing rate in tracheid length of annual rings was estimaled by the regression curve ovtanied, and the juvenile and mature wood could be differentiated at the point of one percent of annual increasing rate in tratcheid length. It. Was considered to he an effective method. The annual rings or distance from the pith of juvenile zone differentiated by this method ranged from 20 to 22 or 12.9- 13.2cm for plantation,from 16 to 18 or 4.1-4.5cm for natural forest of China fir,from 19 to 23 or 8.7- 10.5 for plantation and from 23 to 24 or 2.9-3. 1 cm for natural forest of Japanese larch respectively. The increment of juvenile wood was in direct proportion to the growth rate.