Fatigue crack growth thresholds ΔK^th given in the WRC Bulletin, IIW and BS 7910 are constant values at negative stress ratios. However, the definitions of ΔK_th at negative stress ratios are different: K_max according to the WRC Bulletin and IIW, and K_max - K_min according to BS 7910, where K_max is the maximum and K_min is the minimum stress intensity factors. From fatigue crack growth tests conducted on ferritic steels and aluminium alloys in air at ambient temperatures, the thresholds expressed by K_max - K_min are variable values at negative stress ratios; that is, the threshold increases with decreasing stress ratio. The thresholds expressed by K_max are also variables. K_max decreases with decreasing stress ratio. The thresholds at negative stress ratios are not constant; they are affected by compression stress. Therefore, the thresholds given by the WRC Bulletin and IIW are slightly unconservative. The threshold given by BS 7910 is considerably conservative. A suitable definition of the threshold at negative stress ratios for application in flaw evaluation procedures is the variable threshold ΔK_th expressed by K_max - K_min.