Polyimide (PI) and PI nanocomposite fibers containing different amounts of multiwalled carbon nanotubes (MWNTs) were produced for the first time by electrospinning. The membranes prepared were composed of highly aligned nanofibers and showed significant enhancement in mechanical properties, compared with the membranes prepared by conventional solution-casting method. Surface-functionalized MWNTs were homogeneously dispersed and highly aligned along the fiber axis, whereas most of the pristine MWNTs formed aggregates or bundles and even protruded out of the electrospun nanofibers. The thermal and mechanical properties of polyimide matrix were significantly improved with the incorporation of MWNTs. And the elongation at break of the nanofiber membranes can reach 100% for the nanotube loading level of 3.5 wt %. It was found that electrospinning the in situ prepared MWNT/poly(amic acid) solution can achieve better polymer chain orientation and thus better mechanical properties of the as-prepared membranes. Our study demonstrates a good example for the preparation of high-performance polymer/carbon nanotube nanocomposites by using electrospinning.