This paper reports a facile method for preparation of modified ultrafine anatase titanium dioxide (TiO2) nanoparticles using the hybrid nanofiber as a microsized reactor. The hybrid nanofibers comprised of poly(vinyl pyrolidone) (PVP) and amorphous TiO2 nanoparticles were prepared via the electrospinning method. When the hybrid nanofibers were treated with water vapor at a temperature of 80 degrees C or above, the nanofibers acted as microsized reactors in which the amorphous TiO2 nanoparticles grew and crystallized into ultrafine anatase TiO(2)w nanoparticles. The PVP matrix in this microsized reactor could be regarded as a substrate for the reaction and protected the over-growth of the TiO2 nanoparticles during the reaction. Morphological and structural characteristics of the hybrid nanofibers and TiO2 nanoparticles were studied with scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) method, Raman spectroscopy, X-ray diffraction (XRD) patterns, Fourier transform infrared (FTIR) spectroscopy and thermal gravimetric (TG) analysis. The anatase TiO2 nanoparticles were modified with PVP molecules during the crystallization process, and are expected to be easily mixed with other polymers to fabricate functional textiles.