The purpose of this study is to suggest design principles of open learning platform (OLP) for teacher training based on findings driven by design-based research (DBR, Reeves 2006). SMART Teaching 3.0 (Kim, Kim, Lee, & Lee, 2016) has been designed, implemented and evaluated as a self-directed learning program for English teachers in this study. Various types of context-rich data from 149 participant teachers, such as user statistics, teacher weekly journals, interviews, and post-survey were collected, analyzed, and triangulated. Findings throughout two teacher training implementations yielded six design principles in developing an OLP that adapts a new teacher training model: 1) sufficient open contents, 2) interface considering both PC and mobile learning environment, 3) purpose-driven online interaction space, 4) short and needs-satisfying contents such as observations of teachers’ lesson, 5) technical supports for individual learning and scaffolding, and 6) utilization of OLP for multi-purpose which are informal learning with individual learners; non-formal learning in community of practice; and formal learning as a supplement of officially certified teacher training program. It is hoped that the design principles and application process be referred for reforming teacher training program and future studies.