The present work is amid to investigate the effect of notch root radius and specimen size (thickness B and ligament b) on the apparent fracture toughness K I,app . Series of fracture tests on non standard compact tension CT specimen of AISI 4340 low alloy steel with different values of notch root radius ( =0.08 mm-3 mm) and ligament size/crack length to the specimen width ratio (a/W=0.1-0.9) have been conducted to measure the apparent fracture toughness K I,app . The experimental results show that K I,app depends on and specimen size (B and b). There is a critical notch root radius c under which the apparent fracture toughness becomes independent of the notch radius . Two empirical models have been proposed to transfer the data among specimens of different notch radii and predict the experimental values of K I,app considering the effect of value. Also, Two models have been developed to transfer the data among specimens with different sizes and predict K I,app considering the effect of specimen thickness B, specimen ligament band the combined effect of B and b values. The combined effec -based model is developed by modifying the E 1921 standard equation (that is usually used to predict the thickness effect only). The models predictions are in good agreement with the present work experimental results and results reported in the literature.