Fresh basalt fibers are practically amorphous. Due to high temperature action these fibers have ability to partially crystallize. This form of basalt fibers can be more brittle and their strength can be too low. In this contribution the strength of basalt filament yarns is investigated at room temperature and after tempering to the 50, 100, 200, 300, 400 and 500 degreesC, Structural changes of fibers are identified by scanning electron microscopy. The strength drop of basalt filaments after long term temperature exposition is described by the linear spline nonparametric model.