This study was performed by using dead branches and knot size analysis data of 50 sample trees in 10 sample plots for Larix olgensis Hengy plantations with different ages, different stand density and different sites. The data statistical analysis and matrixes and simulating technique are uses for analyzing and studying the distribution of the knot size along the stem. The models of predicting knot size were established. The results indicate: (1) the knot diameter increases with the knot height and reaches limit value at the tree crown base. The knot diameter can be described with Mitscherlich equation. (2) below 40% of the tree, the sound knot length increases along with the height of living locations of the knot, then the trend tends to decrease.(3) the loose knot length increases with the increasing of tree diameter at breast height and decreases with the increasing of knot angle.