SnO2 nanofibers were synthesized by electrospinning method, and then, treated by oxygen plasma. The morphology and structure of treated SnO2 nanofibers were analyzed using XRD, SEM, and TEM. The composition and specific surface of treated SnO2 nanofibers were analyzed by EDX, XPS, and BET. The treated SnO2 nanofibers showed more pores on the surface of the nanofibers. The size of treated SnO2 nanoparticles in the nanofibers are similar to 20 nm. The porosity of the treated SnO2 nanofibers was higher than that of the untreated. The gas sensing properties of the SnO2 nanofibers to formaldehyde were measured before and after materials treated by oxygen plasma. The treated SnO2 nanofibers exhibit low operating temperature and large response values. Cross-responses of the treated SnO2 nanofibers sensor were tested. The sensing mechanism of the SnO2 nanofibers gas sensor treated by oxygen plasma was briefly analyzed.