To replace the current PbO system of seal frits in the liquid crystal display (LCD) back-light unit (BLU), we examined the thermal behavior of lead-free frit, Bi2O3 glasses. Frits consisting of a major composition of Bi2O3, ZnO and B2O3 were heat-treated and analyzed under different frit sizes using thermal analysis. The variation in frit size affected the crystallization mechanism of the frits during firing. Our findings suggest that devitrification of bismate glass frits is closely related to heating profile and frit size.