Abstract: The fatigue crack propagation rate test has been carried out for the HAZ and weld of 20MnHR steel used in nuclear industry.Compact tension (CT) specimens were used in the test.The number of valid HAZ and weld specimens is respectively 14.In test,(da/dN)_I and (ΔK)_I of each group specimens were computed with seven point incremental polynomial technique.All (da/dN)_I and (ΔK)_I data of 14 HAZ specimens and 14 weld specimens were respectively together to carry on regression analysis.As a result,found the inflective point of crack propagation rate,and brought forward respectively relation of (da/dN)_I and (ΔK)_I by means of the Paris crack propagation formulas based on plane strain behavior and plane stress behavior.The test results and analyses showed that the Paris formulas based on plane strain behavior can be applied to practical welded structures with considerable confidence.