The notion of learner autonomy is one of the major theoretical constructs studied in L2 learning. Drawing on Deci and Ryan's (1985) Self-Determination Theory (SDT), I sought to investigate and describe L2 learner autonomy and how an educational intervention influences it. The SDT conceptualizing human motivation for learning as existing on a continuum from extrinsic to intrinsic motivation provides ways of measuring learner autonomy and a rationale for educational interventions for developing it. First, Japanese adult EFL learners' characteristics were described in terms of learner autonomy-related psychological constructs (motivation, affect, and strategy use) according to levels of learner autonomy based on SDT. Second, the adapted VSS yielded significant effects on the participants' vocabulary learning and L2 learning anxiety (for the high and low autonomous motivation groups) and social strategy use (for the low autonomous motivation group). Third, the implementation of the adapted VSS into the adult L2 English classes was considered in terms of the development of linguistic and autonomous forms of learning quoting from the participants' quantitative and qualitative responses for this approach. / CITE/Language Arts