SnO2-ZnO composite nanofibers fabricated using an electrospinning method exhibited exceptional hydrogen (H-2) sensing behavior. The existence of tetragonal SnO2 and hexagonal ZnO nanograins was confirmed by an analysis of the crystalline phase of the composite nanofibers. A bifunctional sensing mechanism of the composite nanofibers was proposed in which the combined effects of SnO2-SnO2 homointerfaces and ZnO-SnO2 heterointerfaces contributed to an improvement in the H-2 sensing characteristics. The sensing process with respect to SnO2-ZnO heterojunctions is associated not only with the high barrier at the junctions, but also the semiconductor-to-metallic transition on the surface of the ZnO nanograins upon the introduction of H-2 gas.