A proposal has been made for the law of fatigue crack propagation rate and may be expressed as da / dN =Δδ _T /Δδ _i C ₀ (Δ K ) ^{m 0} where da / dN ; rate of fatigue crack propagation Δ K ; range of stress intensity factor C ₀, m ₀; material constant Δδ _T ; variation of crack tip opening displacement from minimum stress to maximum stress in test specimen Δδ _i ; crack tip opening displacement at K _in=Δ K where K _in is stress intensity factor in an infinitely wide plate with a through crack under uniform tension. This equation is in very good agreement with results of fatigue crack propagation test under various stress ratios and with various test configurations. This suggests that C and m in Paris's equation da / dN = C (Δ K ) ^m are affected by load conditions and specimen configurations. It also suggests that a region of accelerated fatigue crack growth leading to rapid terminal failure at high Δ K values, so called “region 3”, might not be appeared in an infinitely wide plate.