Abstract: Low cycle fatigue surface crack propagation rate(SCPR) test of 10CrNiMo steel with four pieces of specimens were carried out,adopting six-grade nominal strain increased gradually as control parameters under cantilever bend loading.It was found that under the control of every-grade nominal strain,SCPR tends to a certain stable value,but local crack propagates sometimes fast and sometimes slow and it is particularly obvious under the control of low nominal strain.Analysis indicated that surface crack propagation presented a zigzag path of transition and bifurcation,which brought on local cracks unstable growth.Usually,transition and bifurcation will decrease surface crack propagation rate,but sometimes also accelerate surface crack propagation.