We report the microstructure and thermoelectric properties of composite nanofiber webs, which were fabricated by dual-electrospinning of polyacrylonitrile (PAN) and sodium cobalt oxide (NaCo2O4) precursor solutions with different input compositions and following heat-treatment at 600-900 degrees C for simultaneous carbonation and calcination. The SEM and EDS mapping images revealed that PAN-derived carbon nanofibers (CNFs) and NaCo2O4-based ceramic nanofibers coexisted in the composite nanofiber webs and that their relative contents could be controlled by the input compositions. The Seebeck coefficient increased from similar to 26.77 to similar to 73.28 mu V/K and from similar to 14.83 to similar to 40.56 mu V/K with increasing the relative content of NaCo2O4 nanofibers in the composite nanofiber webs fabricated at 700 and 800 degrees C, respectively. On the other hand, the electrical conductivity of the composite nanofiber webs increased with the decrement of the relative content of NaCo2O4 nanofibers as well as the increment of the heat-treatment temperature. Owing to the opposite contributions of NaCo2O4 nanofibers and CNFs to the Seebeck coefficient, electrical conductivity and thermal conductivity, a maximum power factor of similar to 5.79 mu W/mK(2) and a figure of merit of similar to 0.01 were attained for CNF/NaCo2O4-based composite nanofiber webs fabricated at 45wt% input composition of NaCo2O4 and at heat-treatment of 700 degrees C-.