The objective of this study was to determine the effect of thinning on the annual ring structure and the cross-sectional dimensions of tracheids in plantation-grown Japanese larch (Larix leptolepis). Annual ring width, earlywood width and latewood width increased significantly after thinning. The width of the band of nonflat latewood tracheids in the annual ring increased more than that of flat latewood tracheids. Thinning did not significantly affect latewood percentage. The average radial diameter of both earlywood and latewood tracheids increased after thinning. After thinning, average wall thickness of earlywood tracheids increased, while that of latewood tracheids decreased. Cell wall percentage in earlywood was not influenced significantly by thinning, but latewood cell wall percentage decreased. The changes of the average radial tracheid diameter, the average wall thickness of tracheids and cell wall percentage from earlywood to latewood within a growth ring became more gradual after thinning. However, thinning did not affect significantly the cell wall percentage of the whole growth ring. This study suggests that thinning has little effect on wood density of the whole growth ring in Japanese larch.