The test results for deriving of the dependencies of critical stress intensity factors on significant agents are presented for glued laminated timber. The dependencies of critical stress intensity factors on moisture content, width of cross section for I and II forms of fracture are investigated. For the second fracture form the influence of glue joint thickness within the limits of 0.07 - 0.35 mm was estimated. The influence on critical stress intensity factor was estimated also for the moisture content of wood within the limits of 12-35%, for crack front width within the limits of 30 - 140 mm, for loading velocity within the limits of 16 - 32 kN/min, for the form of crack tip, and the location of crack - in glue joint or in wood.