Low-humidity sensing performance of raw multi-walled nanotubes (MWCNTs), graphene and carboxylic acid functionalized MWCNTs (MWCNT-COOH) and graphene (GO) thin films were investigated by using a quartz crystal microbalance (QCM). The characterizations of the thin films were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Water vapor molecules adsorbed onto the GO thin film had a larger frequency shift than the MWCNT-COOH thin film (especially at low humidity levels). Adsorption dynamic analysis, molecular mechanics calculation (association constant), was applied to elucidate how amounts of surface functionalized groups (COOH) of MWCNTs and graphene increase the sensitivity to low-humidity.