The purpose of this study was to investigate pre-service science teachers' understanding levels of Kepler's second and third laws. The participants of the study were 38 pre-service science teachers who attended introductory astronomy class in their teacher education program. The participants' understandings of Kepler's laws were examined by their answers to ranking tasks, which required participants to rank the situations given in the question, and then to explain the reasons behind their answers. The findings showed that the participants' understanding levels ranged from partial understanding with alternative conceptions to sound understanding. Moreover the number of the participants with partial understanding with alternative conceptions exceeded the participants with sound understanding. The participants' explanations to ranking tasks also indicated that although participants knew the classic statements of Kepler's laws, they had also alternative conceptions. Five different alternative conceptions were identified from the participants' explanations. Two of them have not been reported in previous studies.