This paper describes studies on wind-duration of strong wind, which is one of the factor for analysing fatigue damage of structures sujected to repeated wind load. In this paper, observed wind data, which is record with duration 72 hours at the center of which annual maximum wind speeds occurs, were used to estimate duration and probability of wind velocity. Generalized probability distribution function of wind velocity were proposed. Duration of strong winds presented as a function of its maximum wind velocity. From these results, multipling generalized probability distribution function by generalized number of (wind velocity) samples, approximate equation of wind-duration was derived. Approximate equation of wind-duration proposed in this Papers useful for analysing fatigue damage of structures under strong wind in Japan.