AbstractAn experimental study was conducted to investigate the effect of size on the strength properties of Chinese larch (Larix gmelinii) dimension lumber. 7546 pieces of dimension lumber were sampled in three sizes, 40 by 65 mm, 40 by 90 mm, and 40 by 140 mm. After visually-grading and grouping, mechanical properties of bending strength, tensile strength, and compression strength parallel to grain were measured in a full-size test. Using nonparametric estimates, the combined length and width size effect parameters of H and L were 0.21 and 0.23 for the bending strength. The width effect parameters were 0.29 in H and 0.33 in L for the ultimate tensile strength parallel to grain. The width effect parameters of H and L were 0.12 and 0.20 for the ultimate compression strength parallel to grain. These size effect factors between H and L could be used when using the lumber for practical purposes.