Self-directed learning with mobile technology (SDLMT) is critical to students' learning success. However, only minimal research has been conducted on the manner by which significant aspects (e.g., selfefficacy, student engagement) are related to SDLMT. This study analyzed the answers of 485 Chinese students (seventh to ninth grades) who were surveyed, and evaluated the relationships among self-efficacy (Internet selfefficacy and online communication self-efficacy), student engagement (behavioral, cognitive, emotional, and social engagement), and SDLMT. Structural equation model revealed that SDLMT was positively predicted by self-efficacy, and SDLMT was positively impacted by student engagement. Moreover, the mediation analysis was performed using the PROCESS plugin in SPSS and determined that after controlling for gender and grade, the relationship between self-efficacy and SDLMT was partially mediated by student engagement. Findings established the value of self-efficacy for SDLMT, and further emphasized the vital mediating role of student engagement. Hence, developing K-12 students' SDLMT entails enhancing self-efficacy and student engagement.