Ultrafine particle (i.e., smaller than 100 nm) in the ambient air is a significant public health issue. The inhalation and deposition of ultrafine particles in the human airways can lead to various adverse health effects. Loose-fitting types of masks are commonly used by the general public in some developing countries for protecting against ultrafine particles in the ambient environment. This research conducted a series of laboratory chamber experiments using two sets of particle sizers and two mannequin heads to study the mask efficiency of selected loose-fitting masks. Results acquired demonstrated that the cloth mask showed a low mask efficiency against ultrafine particles with the mask efficiency generally less than 0.4. The KN95 presented a better mask efficiency among all tested masks with the mask efficiency overall larger than 0.5. In addition, the effect of mask-wearing on the change of ultrafine particle airway deposition efficiency was also investigated in this study. The ultrafine particle deposition efficiency in the airway section studied was found to decrease due to mask-wearing, and the decreases of the deposition efficiencies were similar among all loose-fitting masks tested.