Fracture toughness under plane stress is used to describe the ability of thin metallic sheet to withstand unstable crack propagation. However, it shows strong sensitivity to the specimen geometry during tests. In this study, the experiment to obtain K-R resistance curves of a typical type of aviation materials, 2524-T3 aluminum alloy sheet, was conducted using middle crack tension specimen with different width and initial crack length and hence determine the plane stress fracture toughness K_c and apparent fracture toughness K_app. The results showed that the K-R resistance curves highly relevant to specimens’ width but less to the initial crack size. The fracture toughnesses and apparent fracture toughnesses are affected apparently by both the specimen width and the initial crack length. An apparent fracture toughness criterion including the width effect is proposed to predict residual strength. Meanwhile, with close observation of the role of “flow stress” (the mean of yield stress and ultimate stress) in the final fracture of cracked cross-section, a model for residual strength evaluation was put forward based on the net section yielding criterion. This model exhibits both net-section yielding and stable ductile tearing during the fracture of ductile metals.