Abstract: Fatigue crack growth threshold values of 2124 aluminum alloy in TL and LT directions were tested. A method was proposed to estimate fracture toughness K′C of 2124 aluminum alloy in TL and LT directions by the maximum load in last 10 cycles in the instable expansion stage of fatigue cracks. The estimated K′C were compared with fracture toughness KC predicted using forman formula. The results show that the values of K′C were close to the values of KC, and the error dispersion was small. Fracture toughness K′C values were brought into the full range fatigue crack growth rate expression to calculate the experiment data of 2124 Al alloy in TL and LT directions, and predicted results were fitted well to the experimental results.