Flexural properties of larch（larix gmelini） dimension lumber（40mm×65mm×4000mm, 40mm×90mm×4000mm and 40mm×140mm×4000mm, Ⅰc and Ⅲc grade） were tested by using the method of America society for testing and materials ASTM D4761 05. The purpose of the present study is to calculate the size effect parameters of modulus of rupture for the dimension lumber of larch when a span to depth ratio of 18∶1 was adopted for bending specimens. The results indicated that the size effect parameters is different between grades. For Ⅰc grade the size effect parameters is 043 and for Ⅲc grade lumber the relation between size effect parameters and strength level showed linear relationship.