Abstract: Background and Objective: In experiments using laboratory animals, Natural Plant Extracts (NPE) containing sweet flag have been found to have anti-atopic and hair growth effect. However, whether NPE could clinically prevent hair loss in people with alopecia is unknown. Therefore, the objective of this study was to determine whether NPE could prevent hair loss in people with alopecia. Methodology: The NPE was applied to a total of 8 subjects with alopecia for 10 months. When the time duration of NPE application was increased, the number of hair loss from subjects was decreased, while the percentage of hair loss prevention was significantly (p<0.05) increased 82.67% compared to before NPE application. Results: At 60 days after applying NPE, 80.24% of subjects had great satisfaction on its effect on hair loss prevention. The density and thickness of hair were significantly (p<0.05) increased to 128 and 233% respectively, compared to before NPE application. Conclusion: These results implies that NPE containing sweet flag could prevent hair loss in people with alopecia.