•Conversion method considering pre-strain direction is proposed.•Fracture toughness tests were performed to investigate pre-strain effect.•Fracture toughness decreases by half or more by 1% LC strain. Previous studies presented the fracture toughness evaluation values decrease due to pre-strain by local compression. However, it was not possible to distinguish whether the cause was fracture toughness deterioration or poor crack front shape straightness due to a bowing crack front at low strain. The authors applied pre-strain without affecting the crack front shape and conducted fracture toughness tests showing that 1% pre-strain reduces fracture toughness by 50%. Furthermore, we investigated the effect of the pre-strain direction on fracture toughness, and proposed a method for converting longitudinal pre-strain to thickness direction strain and demonstrated its effectiveness.