Due to special characteristics of nanofibers such as nanometer sized diameter, high surface area to volume ratio and higher alignment of molecular chains, nanofibers are expected to be widely used in many applications such as filter medias, drug delivery systems and scaffolds. Nanofibers containing magnetic nanoparticles have potential uses as functional nanofibers for many applications such as sensors and materials for drug delivery systems. As a material of nanofibers, due to their biocompatible and nontoxic characteristics, polylactic acid (PLA) and magnetite (Fe3O4) can be used. In the fabrication process of nanofibers containing magnetic nanoparticles, controlling nanoparticle dispersion in nanofibers is one of the important issues to be overcome, and the effect of nanoparticle dispersion on the mechanical property of nanofibers has to be evaluated. In this study, three types of surface treatments, oleic acid treated, stearic acid treated and hot water treated are conducted to disperse magnetite in nanofibers. Random and aligned PLA nanofiber fabrics and magnetite/PLA composite nanofiber fabrics were fabricated by the electrospinning method, and their mechanical properties were revealed by tensile test. Oleic acid treatment was the most effective surface treatment. Highly aligned nanofibers can be fabricated by using a high rotation speed of drum collector. The tensile strength of PLA nanofibers and oleic acid treated magnetite/PLA composite nanofibers shows almost same values, although the other treatment has the lower tensile strength than PLA nanofibers. Aligned nanofibers tended to show high strengths.