Abstract: The self-thinning rule, a universal biological law in an even-aged plant population, describes decreasing processes of population densities at -3/2 power of plant mean weight on log-log graph. In the paper, the author introduced the basic contents of the rule, discussed effects of site and shading on the rule, and proved theoretically that the power of the rule must be larger than an abs?lute value of -3/2 in the tree population. The author also noted the relations between the densities at the density-dependent mortality stage and at the early growth stage of population. The biological and cclolgical fas tors influencing the process of the self-thinning were discussed.