Abstract: The expressions of threshold value △K_th of fatigue crack propagation in base metal, weld metal and heat affected zone (HAZ) for 09CuPCrNi steel under various values of stress ratio have been obtained respectively by using gradual down K technique according to Chiness Standard GB6398-86. It was found that threshold values in the three parts of a welded joint would decrease with increasing stress ratio R when R≤R_c.th, whereas the values would remain constant when R>R_c.th. Meanwhile a calculating method for stress intensity factor K_c1 of crack closure has been described in detail. It was also indicated that the closure effect of the welded joint is greater than that of the base metal. The analysis and explanation for this viewpoint have been made.