Three groups of Dahurian larch（Larix gmelinii） dimension lumber with different dimensions of 40 mm × 65 mm × 4000 mm,40 mm × 90 mm× 4000 mm,and 40 mm × 140 mm× 4000 mm,were visually graded according to Chinese Standard GB 50005-2003.The main defect causes that degraded the lumber were identified for each group.The results showed that:1) different sizes of lumber differentiated in proportion to each grade of dimension lumber;2) knots,shake/checks and skips were the main defects that lowered the lumber grade.It was proposed that further study should be conducted toward optimizing processing and drying methods to improve the quality of dimension lumber.