The purpose of this study was to assess the changes in the chemical composition and thermal properties of Brutian pine (Pinus brutia Ten.) fibers treated with 0.25%, 0.50%, 0.75% and 1% NaOH solutions and their effects on the physical and mechanical properties of produced fiberboards. Main chemical components and Fourier transform infrared (FTIR) spectroscopy analyses revealed that the amount of extractive substances, lignin and hemicellulose decreased constantly as the NaOH concentration increased in alkali treatments, while the cellulose amount increased constantly. Thermogravimetric analysis (TGA) results showed that alkali treatment decreased the thermal resistance of the fibers. Alkali treatment of fibers increased the water absorption (WA) and thickness swelling (TS) of fiberboards. As the concentration of the alkali treatment increased, the mechanical properties of the fiberboards (the modulus of elasticity (MOE), the modulus of rupture (MOR) and internal bond strength (IB)) were also increased. The boards produced from untreated fibers and fibers treated with 0.25% and 0.50% NaOH were met the requirements of the standard TS EN 622-5 (2011) in terms of TS, MOE, MOR, and TB strength for load-bearing boards to be used under dry conditions.