The plastic deformation zone size near the propagating fatigue crack for SM41 steel were observed by optical interference method. The effect of plastic zone size on crack propagation rate due to a single tensile overload were investigated. Main results obtained are as follows; 　(1) The plastic deformation zone size increase with a growth of fatigue crack. (2) The relation between the plastic deformation zone size (Wx) and the ratio of ΔK/σy for SM41, appeared as Wx=0.18(ΔK/σy)2. (3) Observed plastic deformation zone size (ρ1) due to a single tensile overload agreed comparatively well with the affected zone size (ρ2).