This paper proposes an estimation technique of fatigue crack length for an aluminum 6061-T6 plate structure with a notch using a stress-life (S-N) curve. First, an S-N curve is obtained from multiple specimen tests by applying constant-amplitude cyclic loading with different stress levels. Next, the parameters of the Paris' law are estimated from the S-N curve by solving an optimization problem. Then, based on the Paris' law, the current crack length is continuously estimated by counting the elapsed fatigue loading cycles. The novelty of this study lies on the followings: (1) estimation of fatigue crack length using only the S-N curve for online fatigue crack monitoring, (2) estimation and experimental validation of the Paris' law parameters estimated from the S-N curve, and (3) experimental validation of the crack length estimation technique for notched aluminum 6061-T6 plates with various thicknesses. (C) 2018 Elsevier Ltd. All rights reserved.