Background: Health behavior including Food consumption of Thai people has changed due to economic and social changes, resulting in health conditions, eventually. Unexpectedly, the biological disaster, COVID-19, has been a cause of death for millions of people around the world, especially people with health conditions. To avoid having a health condition, we could have good health practices such as eating good foods, getting enough rest, taking exercise, etc. Objective: To assess Nutrition and Immunity related knowledge, and Health Behavior during COVID-19. And to study factors affect Health Behaviors of participants. Method: Cross-Sectional Study was used to gather information by using questionnaires through an online platform; Google Form. Students from grades 10 to 12 who could access the internet participated in the questionnaires. There were 138 students who participated in this study. Result: From the study about Nutrition and immunity knowledge and health behavior during the COVID-19 pandemic, there were 138 participants. Most participants illustrated Nutrition and Immunity Knowledge at a low score (M=5.63, SD=1.35), for Health Behavior score, most participants showed a moderate level of score (M=66.78, SD=11.85). Taking vitamins and supplements was a predictive factor for health behavior during COVID-19. Conclusion: The only factor affecting health behavior of participants during COVID-19 pandemic was vitamin and supplement intake during COVID-19. Immunity in our body can be improved in mainly 2 ways 1) taking vaccination when needed 2) having good health behavior such as eating good foods, taking enough rest, and taking exercise. To have people practice good Health Behavior to strengthen immunity, education especially nutrition and immune system knowledge should be provided to the public which can be easily accessed. Keyword: food consumption, immune system, COVID-19