B ackground: E-cigarettes contain various kinds of content, but as with conventional cigarettes, the main content is nicotine, but the difference is in the nicotine content contained in conventional cigarettes and e-cigarettes. The nicotine content in e-cigarettes is known to be lower than conventional cigarettes. Other ingredients found in e-cigarette liquids or liquids are propylene glycol, flavours, glycerin, and water. Halitosis is defined as bad breath coming out of the mouth that can involve a person's health and life. Halitosis is one of the main and oldest problems that make it difficult to socialize. Objective: The purpose of this study was to determine the level of knowledge of dental preclinical students regarding e-cigarettes against halitosis. Methods: This study is a descriptive observational with a cross-sectional design to determine the level of knowledge of dental preclinical students about the effect of e-cigarettes on halitosis. Results: Results: The results showed that a small proportion of respondents had moderate knowledge about e-cigarettes, namely 39% who answered agree and 10% strongly agreed. While the knowledge of respondents about halitosis is very large, as many as 68% who answered agree and strongly agree as much as 17%. The level of knowledge about e-cigarettes on halitosis is closely related to the level of smoking habits. The importance of knowledge that a person has will affect awareness in smoking. Conclusion: Students of the Faculty of Dentistry, Trisakti University class 2018-2020 partially understand the effect of e-cigarettes on halitosis.