The La2CuO4 crystal nanofibers were prepared by using single-walled carbon nanotubes as templates under mild hydrothermal conditions. The steam reforming of methanol (SRM) to CO2 and H2 over such nanofiber catalysts was studied. At the low temperature of 150 °C and steam/methanol=1.3, methanol was completely (100%, 13.8 g/h g catalyst) converted to hydrogen and CO2 without the generation of CO. Within the 60 h catalyst lifespan test, methanol conversion was maintained at 98.6% (13.6 g/h g catalyst) and with 100% CO2 selectivity. In the meantime, for distinguishing the advantage of nanoscale catalyst, the La2CuO4 bulk powder was prepared and tested for the SRM reaction for comparison. Compared with the La2CuO4 nanofiber, the bulk powder La2CuO4 showed worse catalytic activity for the SRM reaction. The 100% conversion of methanol was achieved at the temperature of 400 °C, with the products being H2 and CO2 together with CO. The catalytic activity in terms of methanol conversion dropped to 88.7% (12.2 g/h g catalyst) in 60 h. The reduction temperature for nanofiber La2CuO4 was much lower than that for the La2CuO4 bulk powder. The nanofibers were of higher specific surface area (105.0 m2/g), metal copper area and copper dispersion. The in situ FTIR and EPR experiments were employed to study the catalysts and catalytic process. In the nanofiber catalyst, there were oxygen vacancies. H2-reduction resulted in the generation of trapped electrons [e] on the vacancy sites. Over the nanofiber catalyst, the intermediate H2CO/HCO was stable and was reformed to CO2 and H2 by steam rather than being decomposed directly to CO and H2. Over the bulk counterpart, apart from the direct decomposition of H2CO/HCO to CO and H2, the intermediate H2COO might go through two decomposition ways: H2COO=CO+H2O and H2COO=CO2+H2.