Abstract: According to the fracture criterion of crack tip strain ε_max=ε_c,It is known that the fracture criterion should be the same for smooth specimen and specimens with crack or notch.So among these three different specimens there must be an internal relation for thier fracture toughness measured.Through generalized analyzing of strain field atcrack tip backed up with results from experiments,the authors established the expression between the opening displacement of crack and that of notch by applying the concept of relaxation slip.Besides,the empirical relationship that P_f has nearly the same magnitude as the average interval λ of the impurities or the second phase particles in material is also obtained,based on the study of the physical nature of the equivalent radius P_f of the crack.Thus the conversion relationship between the fracture toughness of notch specimen and that of crack specimen could be written as((δ_c)_f)/((δ_c)_n)=((δ_pc)_f)/((δ_pc)_n)=((δ_ec^*)_f)/((δ_ec^*)_n)=√(ρ_f/ρ_n)≈√(λ/ρ_n),which has been preliminarily verified by experiments.