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In this research, a general mixed mode I/II fracture criterion is developed for fracture investigation of orthotropic materials. Various experimental tests show that cracks always propagate in an isotropic medium and along fiber direction in orthotropic materials. With a novel material model titled an Equivalent Reinforced Isotropic Model (ERIM), fracture criterion can be extended for investigation of fracture in orthotropic materials. This inspires that fracture in orthotropic materials follows the fracture mechanism in isotropic materials. This new criterion is developed based on extension of MTS which is widely used for isotropic materials. Also in this research the effects of T-stress in fracture of some specimens has been studied. A comparison between available experimental observations and theoretical estimation implies on capability of developed criterion for predicting both crack propagation direction and fracture instance, wherein the achieved fracture limit curves are also compatible with fracture mechanism of orthotic materials. It is also shown that non-singular T-stress term has a significant impact on orthotropic material failure, especially when the second mode is dominant mode. It is shown that unlike isotropic materials, fracture toughness of orthotic materials in mode I (K_IC) cannot be introduced as the maximum load bearing capacity and thus new fracture mechanics property, named here as maximum orthotropic fracture toughness in mode I (├ K_IC ┤|ortho) is defined. Considering ease of access, wood is used as experimental specimen for the purpose of comparing the results.