It is important to cultivate dispositions of 21st-century skills (D21S: critical thinking, complex problem solving, creativity, communication, and collaboration) in project-based science, technology, engineering, and mathematics learning (STEM-PjBL). Previous research has seldom adopted the person-oriented approach to examine STEM-PjBL program effectiveness, and knowledge regarding group differences is still lacking. To fill this gap, a person-oriented analysis was used to form groupings of students based on D21S. Three latent profiles (high-, medium-, and low-D21S) of students were identified in 6 elementary schools, and the transition probabilities were estimated for 2 class types (treatment vs. comparison) across 1 semester. Most of the students (63.4%) showed stable D21S profiles over 1 semester; however, some students (17.6%) exhibited improvement, and some (18.9%) showed declination. Additionally, the STEM-PjBL programs presented positive effects on the growth of D21S. This study further found that the D21S transitional types (i.e. declined, stable, and improved) could predict students' motivation (perceived STEM domain content mastery, self-efficacy, and task value), showing that a declined D21S also decreased academic motivation. The findings suggest that students who decline in D21S should receive attention from teachers because their perceived academic performance, competence beliefs, and values in STEM also decrease simultaneously.