Nanoscale flows appear widely in nanofiber fabrication especially in electrospinning, where a nanoscale jet is a multi-phase fluid including solvents and additives. This paper shows that the fiber morphology can be effectively controlled by adjusting the density of the multi-phase jet. TiO2 is used as an additive to produce polyacrylonitrile nanofibers by electrospinning; the results elucidate that uniform nanofiber can be obtained, and this phenomenon is explained theoretically.