In this study, pristine carbon nanotubes (CNTs) and the CNTs functionalized through heat treatment were fabricated, and their hydrogen (H-2) gas sensing characteristics were evaluated at room temperature. Pristine CNTs exhibited relatively low reactivity toward H-2 gas, whereas functionalized CNTs demonstrated linear electrical response characteristics with an increase in the gas concentration due to the presence of functional groups (such as COOH and OH) on the CNT surface, which reduced the activation energy for the adsorption of H-2 molecules and provided additional reactive sites. Thus, functionalized CNTs can be potentially utilized in various fields because their operation does not require high-temperature environments (unlike the currently used metal oxide-based H-2 sensors), while the power consumption and explosion risks of these materials are easily reduced through a simple heat treatment process.