Objectives: this study aimed to assess the knowledge of medical students in ionizing radiation and to study the effect of a 3-hour lecture in correcting their misconceptions. Methods: a cohort study was conducted on medical students at Almaarefa Colleges, Riyadh, Kingdom of Saudi Arabia during the academic year 2015-2016. A 7-question multiple choice test type questionnaires administered before and after a 3-hour lecture was used to assess their knowledge. The data were collected from December 2015 to February 2016. The lecture was given to 333 (72%) participants, out of the total of 459 medical students. It covered topics in ionizing radiation and radiation protection. The questionnaire was validated and analyzed by 3 content experts. Results:  of the 333 who attended the lecture, only 253 (76%) students completed the pre- and post questionnaire and they were included in this study. The average student score improved from 47-78% representing a gain of 31% in knowledge (p =0.01). Conclusion:  the results indicated that the medical student's knowledge regarding ionizing radiation and radiation protection is inadequate. Additional lectures in radiation protection significantly improved their knowledge of the topic and correct their current misunderstanding. This study had shown that even with one dedicated lecture, students can learn and learn general principles regarding ionizing radiation.