The purpose of this study is to explore the children's understanding of moon phase changes. Study methods include qualitative and quantitative methods. The subjects of the study are 683 students from grade three to six in an elementary school in New Taipei City. This study uses the method of writing assessment to explore the ideas of elementary school children on the causes of the moon phase changes and analyze the explanations of the causes of the moon phase changes. Students' writing assessment were collected as the major source for the study. In the analysis of the study data, first evaluate and classify the content and explanation of the causes of the moon phases by the schoolchildren, then conduct descriptive statistics to analyze the frequency distribution and percentage, and compare them according to the grade level. The writing assessment processes and the reliability and validity of the study were confirmed by three science educators and senior teachers. The results of the study found: (1) When students explain the causes of the moon phases of the full moon, waxing moon, last quarter moon, and first quarter moon during the day, the 3th graders mostly consider their own relationship, while the 4th, 5th, and 6th graders hold more views of the sun, moon, and the earth; (2) In terms of the proportion of students that the formation of moon phases is related to the sun, except that the proportion of last quarter moon in the 5th grade is slightly higher than that in the 6th grade, the rest are the highest in the 6th grade, indicating that the higher grade students feel the sun more related to the formation of the moon phases. [For the full proceedings, see ED623149.]