Samples of Granulated blast furnace slag (GBFS) from Formosa steel plant (called as Formosa GBFS) were collected and used for several testing series regarding potential application as an alternative material in construction and civil works. Un-hydrated GBFS particle shows river sand-like shapes by all appearances with more angular and therefore, it shows higher shear strength than those of natural sand. In addition, GBFS has latent hydraulic characteristics affecting its physico-mechanical properties when working in wet conditions, especially alkaline ones. In this study, specimens were cured in seawater from 0 day to 280 days under the air temperature both in-room and outdoor. After curing, hydrated specimens were tested for unconfined compressive strength and hydration reaction ratio. It is shown from the experimental results that the unconfined compressive strength and the hydration reaction ratio of Formosa GBFS increase with curing duration and such a tendency is slightly affected by curing temperature between indoor and outdoor. An estimation method of the hydration reaction ratio and the hydration-induced strength was developed for Formosa GBFS cured in seawater.