Background: Nano-sized water particles have been thought to moisturize the skin more effectively. However, clear benefits of humidifier generating nano-sized water particles on the skin have not been studied. Objective: The aim of this study is to examine the effects of humidifier generating nano-sized water particles on the skin by measuring the levels of the skin hydration state and skin barrier function with an objective, quantifiable method. Methods: A 4-week, randomized, case-control study was conducted in 40 healthy Korean women, aged between 20 and 39, and they were divided into two groups, the experimental and control groups. The experimental group used humidifier generating nano-sized water particles for 8 hours every night, during 4 weeks. Skin hydration and transepidermal water loss (TEWL) were measured every week on the forehead and cheek using corneometer and tewameter, respectively. Safety evaluations were also performed at each visit. Results: The baseline skin hydration and TEWL values showed no significant differences between the two groups. After 1 week of use, the experimental group showed significantly increased skin hydration values (p<; 0.001, p < 0.0001) and decreased levels of TEWL values (p=0.017, p=0.025) as compared to the control group. During a 4-week study period, increased skin hydration and decreased TEWL were sustained in the experimental group. No adverse effects were observed in all the volunteers. Conclusion: These results show that humidifier, which generates nano-sized water particles, seems to positively affect skin hydration and skin barrier function. More studies and sufficient follow-up time are needed for the assessment of the humidifier generating nano-sized water particles.