In this paper, phenomenon of fatigue crack propagation is considered as continuous fatigue crack initiation at crack tip. It is assumed that fatigue crack initiation life is used up when damage caused by cyclic strain near crack tip is accumulated and magnitude of the damage is reached to a certain critical value at Δ N strain cycles and size of the crack length Δ a is average grain size of material. Using the above assumptions and proposed cumulative damage law, crack propagation rate da/dN under fully reversed loading is expressed by the following equation. da/dN =Δ a /Δ N = d /ε_f-2ε₀Δε ^α _p where Δε_p : cyclic plastic strain range at a point situated Δ a apart from crack-tip ε _f : monotonic true fracture strain ε₀ : maximum strain at 1 st cycle d : average grain size α : exponent in equation Δε ^α _p · N _c =constant The above equation can be transformed in to the following form. da/dN = d /ε _f -2ε₀ [Δ K / E √2π d {(Δ K /2σ _{Y '}√2π d -1) ^Z - (Δ K /2σ _{Y '}√2π d -1) ^{- Z} }] ^α where Δ K : range of stress intensity factor E : Young's modulus Z : (1- n ) / (1+ n ) n : exponent in equation of cyclic stress-strain curve with strain hardening effect σ _{Y '} : yield stress in the cyclic stress-strain curve Further, the above equation may be expressed by the well known power low formula approximately in the range where Δ K is 50 kg·mm^-3/2500 kgmm^-3/2. da/dN = C (Δ K ) ^m C = d /ε _f {1/ E √2π d (1/2σ _{Y '}√2π d ) ^Z } ^α m = (1+ Z ) αkg·mm^-3/2 There is the following relation between coefficients C and m . log C = A · m + B A=1/1+ Z ·log1/ E √2π d (1/2σ _{Y '}√2π d ) ^Z B =log ( d /ε_f) Crack propagation rates calculated by these equations agree with experimental ones of mild steel quite well, and then these equation may be used for estimation of crack propagation life with good accuracy.