In the process of the fracture initiation from a precrack, plastic blunting or, metallographically, stretched zone is formed at the notch tip. Although this plastic blunting is considered to be favorable because of its effect of relaxation of stress concentration, large plastic strain to be formed around the blunted notch tip will cause strain aging and embrittlement of the notch tip. The effect of this type of strain aging on toughness of steels has been investigated. Such test conditions are chosen as to predict the embrittlement which may occur in actual service and the typical test conditions employed are as follows : preloading to induce 3040μm stretched zone depth (which is 3040% of S. Z. D. at fibrous fracture initiation), aging temperature : 50°C, aging. time : 01000 ^h . Some tests on other conditions have also been made additionally. The test results have shown that SS 41 is most susceptible to embrittlement by strain aging and increase in T_i (the lowest temperature at which fracture occurs by fibrous crack) due to strain aging is 27°C. SM 41 A is also susceptible to strain aging but has shown smaller increase in T_i (increase in T_i due to strain aging ≅17°C) than SS 41. SM 50 C has not shown any indications of embrittlement by strain aging. COD-criterion of preloaded and aged steels has also been investigated and it is shown that COD at fibrous crack initiation, δ _i , is almost constant regardless of preloading and strain aging.