Size effect is one of important factors to adjust tensile strength of dimension lumber to an desirable property. The dependence of the ultimate compression strength on the width of Chinese larch visually graded dimension lumber was investigated. 1 630 dimension lumbers were parted into two groups of high visual grades and low visual grades; three lumber widths(6.5 cm, 9.0 cm and 14.0 cm) were selected and ultimate compression strength property was tested. The width effects of the 5th percentile of ultimate compression strength were estimated by the slope method and the parametric method. Using nonparametric estimates, the estimated width effect parameters of high grade(H) and low grade(L) were 0.11 and 0.20 for the 5th percentile. The parameters of L were larger than those for H. The width effect could be used when using the dimension lumber for practice purposes. When two parameter Weibull distribution functions were fitted to the strength data, the estimated shape parameters of the Weibull distribution by the parametric method were almost identifical to the inverse of nonparametric parameters for the L grade.