This paper reports on an attempt to use reactive extrusion with peroxide as a comfortable pathway for improvement of the crystallization of poly(3-hydroxybutyrate) in a melt spinning process. At first, rheological and thermal properties of the modified melts are determined in order to assess the effect of nucleation. Then spinning tests are carried out. Molecular weights and molecular weight distributions of the spun fibers are determined by chromatographic methods. Average crystallite size is measured by wide angle X-ray scattering. Thermal and textile properties of the spun PHB fibers are also determined. An estimation of the improvement of the crystallization in the spinline and of the inhibition of the secondary crystallization in the fibers from the use of the described way of reactive extrusion is given.