Abstract: The fatigue crack growth (FCG) behavior of the weld heat-affected zone of 14MnNbq bridge steel was systematically investigated in this paper. Firstly, the FCG rates and the threshold values for various stress ratios were obtained via the experimental investigations based on center crack specimens (MT). Secondly, the general expressions for FCG rate and threshold value including the influence of stress ratio were derived. Finally, by assuming the effective stress intensity factor amplitude as , a new method for determining the fatigue crack closure factor by using the threshold Kth were discussed. It is obvious that the FCG formula agrees with the experiments’ data very well.