Co3O4 nanowalls and nanowires have been synthesized onto silicon substrates by low-temperature thermal oxidation of sputtered Co thin films in static air. The synthesis method is very simple and suitable for large-scale fabrication. The effects of the thermal oxidation temperature and duration on the size, amount, and length of the nanowires and nanowalls are systematically investigated both by scanning electron microscopy characterization and differential scanning calorimetry thermal analysis. It is found that the Co/CoO oxidation and Co3O4 decomposition are important factors contributing to the growth of the Co3O4 nanowalls and nanowires. The mechanical adhesion between the Co3O4 nanowalls/nanowires/film and the silicon substrate is observed to be very strong, which is beneficial for many practical applications. Based on the experimental observations, the detailed growth mechanisms of the nanowalls and nanowires are presented. Finally, the promising novel exothermic reaction properties of the Co3O4 nanowalls and nanowires with Al are investigated by thermal analysis.