Fatigue crack growth tests in a wide range of fatigue crack propagation rates covering the threshold condition were conducted at various stress ratios using a quenched-tempered and high strength steel SCM435 with changing heat treatment. The crack closure behavior due to both the plastic deformation at the crack tip and the formation of oxide deposits on crack faces at an ultralow, near-threshold propagation rate were confirmed experimentally by the proposed Top-on ultrasonic method. The influences of stress ratio, strength level and microstructure on fatigue crack growth characteristics were discussed on the basis of the crack closure concept. It was shown that the fatigue crack propagation rate, da/dN, can be related uniquely to the effective stress intensity factor range, ΔKeff, and the threshold level defined at da/dN=10-8mm/cycle in terms of effective stress intensity factor range, ΔKeff, th, are nearly constant regardless of stress ratio and strength level. However, the relationship between da/dN and ΔKeff at such a high stress ratio as R=0.8 where the plasticity induced crack closure and the oxide induced crack closure don't exist, didn't agree with the above relationships at R=0.05, 0.15, 0.4 and 0.6, and ΔKeff, th was Large.