This study investigated the low-humidity sensing performance of raw multi-walled nanotubes (MWCNTs), oxidized MWCNTs (MWCNT–COOH) and Pd-modified MWCNT–COOH (Pd–MWCNT–COOH) thin films by using a quartz crystal microbalance (QCM). The characteristics of the thin films were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Water vapor molecules adsorbed onto the Pd–MWCNT–COOH thin film had a larger frequency shift than raw MWCNTs and MWCNT–COOH thin films. Adsorption dynamic analysis, molecular mechanics calculation (association constant), was applied to elucidate how surface modification and functionalization of MWCNTs increase the sensitivity to low-humidity.