The features of physical and mechanical properties of trees (Pinus sylvestris L. and Larix sibirica Ledeb.) are investigated in experimental mixed forest plantations created by sowing in the Irkutsk region. Saw cuts were taken in the butt part of model trees to determine the properties of wood. The studies were carried out according to generally accepted methods on certified equipment. According to the research it has been found that the average width of the annual layers of pine and larch is 3.54 mm and 2.93 mm, respectively. The average number of annual layers in one centimeter is from 3.42 to 3.61 mm, which is 3.24 and 3.73 times less than the average statistical data for these tree species. It allows us to conclude: the width of the annual layer is significantly greater in the first 20 years of forest plant life than in mature state. Strength indicators of the studied samples of pine and larch are lower by 12-30 % than standard ones. During mechanical tests, wood defects were found in the butt part of the larch, which led to the destruction of the specimens. These defects were dormant bud marks and bulging butt, which form at a fairly early age. Conditional density of pine and larch was determined using the Resistograph 4450 instrument for an additional qualitative assessment of wood. As a result of the studies, it can be concluded that strength characteristics of pine and larch in mixed plantations are quite high.