Phenomenological theories on the effect of pulp fiber properties on the fracture energy of paper are discussed. The effect of fiber length and strength is clarified experimentally. Fiber length appears to affect fiber failure probability only slightly. When fiber strength is changed, the fracture energy decreases greatly with only a small increment in fiber failure probability. This suggests that the fracture energy contribution of a fiber may be correlated between fibers. The effect of fiber length and strength on the cohesive stress – crack widening relationship is clarified.