Teachers' beliefs such as pedagogical beliefs, self-efficacy, and technology value beliefs are influential to technology integration practice. This study aims to investigate teachers' beliefs and the impact on their 3D printing integration in science classrooms. A total of 26 K-12 teachers across six states in the U.S. participated in a nationally funded project. Teachers' STEM education lesson plans were analyzed to assess their 3D printing and STEM integration levels. Teachers' beliefs were collected through a survey with rating scales adapted from previously validated surveys and several open-ended questions. Correlation analysis was conducted to investigate how teacher beliefs were associated with their 3D printing integration. Thematic analysis of the open-ended questions provided a detailed view on teachers' experiences and perceptions, which further explained teachers' beliefs and the impact on their 3D printing integration practice. This study revealed that teachers' pedagogical, self-efficacy, and technology value beliefs were generally not correlated with their 3D printing integration practices except for a negative correlation between teachers' self-efficacy in pedagogical content knowledge and their STEM integration levels. Teachers perceived 3D printing integration as beneficial for students, but they encountered a number of challenges including logistic and technical issues, lack of time and resources, insufficient ability to use 3D printers and connect 3D printing with curriculums, and challenges in teaching students with individual differences. Implications for practice and future research are discussed.