This study was grounded in the social cognitive career theoretical framework (Lent, Brown, & Hackett, 1994). The purpose of this four-year longitudinal study was to examine the factors that may have contributed to students' motivation to develop STEM interest during secondary school years. The participants in our study were 9th-11th grade high school students from a large K-12 college preparatory charter school system, Harmony Public Schools (HPS) in Texas. We utilized descriptive statistics and logistic regression analyses to carry out the study. The results revealed that three-year survey takers' STEM major interest seemed to decrease steadily each year. Although there was a significant gender gap between males and females in STEM selection in 9th and 10th grade, this difference was not significant at the end of 11th grade. White and Asian students were significantly more likely to be interested in STEM careers. We also found that students who were most likely to choose a STEM major in college had higher parent and teacher expectations, higher math and science self-efficacy, higher GPA grades, took more AP courses, and participated in STEM clubs.