In Siberian larch, shear strength (SS) is lower in sapwood (sW) than in mature heartwood (hW) despite the similarity of their cell structures. In sW, SS was not correlated with other properties, not even with density. The aim of the present study was to find the parameters affecting the SS in sW. The diameters of earlywood (EW) and latewood (LW) tracheid lumens [radial diameter (RD)], as well as their double wall thicknesses (2CWTs), were measured from different wood types. The 2CWT beside the resin canals was measured and rays were counted. The wall:lumen ratio of tracheids was calculated. None of the measured and calculated characteristics significantly correlated with SS in hW, while a high 2CWT and wall:lumen ratio of EW increased the SS and a large RD decreased the SS in sW. Tracheids of sW were larger and their walls were thicker than those of mature hW. The EW of sW sheared through tracheids, but practically never through rays, while in the EW of mature hW, both rays and tracheids could be sheared. It is suggested that change in viscoelastic properties, occurring in cell walls during the transformation from sW to hW, improves wood shear properties.