Carbon nanofibers with a multiple tubular porous structure were prepared via electrospinning from a polymer blend solution of polyacrylonitrile (PAN) and polylactide (PLA) followed by carbonization. The electrospun composite nanofibers underwent pre-oxidization and carbonization, which selectively eliminated PLA phases and transformed the continuous PAN phase into carbon, thereby porous structure formed in the carbon nanofibers. The morphologies of as-spun, pre-oxidized and carbonized nanofibers were studied by scanning electron microscope (SEM) and transmission electron microscopy (TEM). It was found that carbon nanofibers with an average diameter about 250 nm and a multiple tubular porous structure were obtained. The chemical changes during thermal treatment were studied by Fourier transform infrared spectrometer (FTIR), Raman spectra, differential thermal analysis (DTA) and thermogravimetric analysis (TG). The results showed that PLA phases were effectively removed and the continuous PAN phase was completely carbonized. The obtained carbon nanofibers had more disordered non-graphitized structures than nonporous nanofibers.