In the present paper a formula for the stress intensity factor (SIF) of a nozzle corner 1/4 circular crack in a P.V. nozzle along the longitudinal section of a cylinder is proposed and verified with the fatigue experimental results of 30-nozzle corner cracks from nine-piece plate-nozzle models and three pressure vessels. The maximum difference between the K(I) calculated with the proposed formula and the experimental results is less than 15%.