This paper introduces the basic principles and methods of X-ray densitometry used at the French National Forestry Research Centre, in which the author had a two-week stay and conducted some experiments under Professor Polge’s guidance.Testing results with Chinese-fir wood (from China) as samples showed that the average annual density changed slightly as the radial diameters of the tree increased and the average annual density differed only by 16.59 percent, while the maximum ring width was 2-18 times larger than the minimum ring width. Thus the author drew a conclusion that the Chinese-fir plants which grow faster would have evener wood quality and an intensive silvicultural system should be adopted to obtain more yeariy increments of the trees.