In this study, the effect of the catalyst pretreatment temperature, which is one of the more important factors in HCHO oxidation at room temperature over Pd catalysts, was examined. To evaluate the effect of the pretreatment temperature on the catalytic activity, Pd-supported catalysts were prepared at temperatures ranging from 673 to 1,073 K under a hydrogen atmosphere. The correlation between the pretreatment temperature and catalytic activity produced a volcano-shaped plot. X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, and CO chemisorption confirmed that the catalytic activity highly depended on the Pd size or dispersion.