This study examines prospective teachers’ levels of understanding the Heisenberg uncertainty principle, and the impact of writing activities for learning purposes on the success of prospective teachers. These prospective teachers study in the Science Teaching Program of the Faculty of Education and take the course Introduction to Modern Physics. In this study, a semi-experimental design with a 'pretest'- 'posttest' control group was used. Research data were obtained via a questionnaire comprising qualitative questions prepared by the researchers. In all the groups, the lesson was taught by utilizing the verbal-written lecture method. In addition, each student in the experimental group wrote a letter to a senior high school student in order to explain the Heisenberg uncertainty principle in an understandable way. On the other hand, students in the control group solved the problems relating to the topic in the course book. A total of 111 third year students studying in the academic year 2007-2008 participated in the present study. 54 of these students were female and 57 of them were male. Research findings indicated that prospective teachers have low levels of understanding the Heisenberg uncertainty principle, and qualitative and quantitative comparisons of posttest results of the experimental group and control group, and success percentages in the written examination were in favor of the experimental group. In addition, the opinions of students about writing activities for learning purposes were determined by means of additional questions to the posttest. 91.4% of the students mentioned that they understood the Heisenberg uncertainty principle about which they wrote the letter. These students also stated that this activity helped in their learning of this topic.