Students` naive theories on the phases of the moon and their origins were investigated. Relationships between students` naive theories and grade level, and relationships between students` naive theories and science achievement level were also studied. Samples consisted of 49 6th grade primary school students, 53 8th grade middle school students, 49 10th grade high school students. Two types of research methods were used: 1) the open-ended written questionnaire: 2) interview. Naive theories were found to persist across grade levels including high school level students. These naive theories involved ① earth`s shadows/obstruction, ② motion of the earth, ③ motion of the moon, ④ combined motion of the earth and moon, ⑤ relative position of the earth, moon, and sun, ⑥ differential reflective properties of the moon and earth, ⑦ changes in weather(clouds), and ⑧ relative distance of the sun and moon. Teachers were listed most frequently as the sources of these naive theories. 6th grade primary school students and 10th grade high school students held more naive theories than scientific theories, whereas 8th grade middle school students held more scientific theories than naive theories. For all science achievement levels, students held more naive theories than scientific theories.