<p>One of the main goals of science education to help students understand scientific concepts in depth. To improve students’ conceptual understanding, it is essential to grasp the main features in their learning process of specific scientific concepts. Therefore, the level division and assessment of students’ conceptual understanding in learning about scientific concepts are crucial. However, there are few studies on analyzing and assessing students’ conceptual understanding of liquefaction. This study explored a level division and a corresponding set of test questions to assess students’ conceptual understanding in learning about liquefaction. A total of 704 seventh-grade Chinese students were evaluated before and after participating in the liquefaction lesson. The results of some quantitative and qualitative analyses show that students’ conceptual understanding of liquefaction can be divided into three different developmental levels, including mechanical memorization of the definition of liquefaction, mastery of the two ways in which liquefaction occurs, and understanding of the nature of liquefaction from a microscopic perspective. This study provides evidence to support the importance of deepening the understanding of the nature of scientific concepts.</p>