Nanosized powders were synthesized via hydrothermal method with different types of surfactants (polyethylene glycol (PEG), cetyltrimethylammonium bromide (CTAB), and sodium dodecylbenzenesulfonate (SDBS)). X-ray diffraction, Raman spectroscopy, and transmission electron microscopy were utilized to characterize the phase structures and morphologies of the products. The sample with CTAB as surfactant (-C) has the largest specific surface area and the smallest particle size among these three samples. The humidity sensor fabricated by -C shows higher performance than those used -P and -S. The impedance of the -C sensor decreases by about five orders of magnitude with relative humidity (RH) changing from 15.7 to 95%. The response and recovery time are 7 and 7 s, respectively. These results indicate that the performance of humidity sensors can be improved effectively by the addition of cationic surfactant.