Self-concept and self-efficacy have been acknowledged as critical predictors of foreign language (L2) learning. However, these two constructs are so closely intertwined that how they function together during L2 learning remains to be determined. The current project aimed to reveal two uncertainties: the bidirectional relationship between L2 self-concept and L2 self-efficacy and their relative effects on L2 achievement. We designed two independent studies: a two-wave longitudinal study to address the bidirectional issue (Study 1) and a correlational study for the relative importance issue (Study 2). For Study 1, we conducted cross-lagged structural equation modeling and found the one-directional relationship from self-concept to self-efficacy. For Study 2, we applied multilevel-structural equation modeling and found that only self-concept significantly predicted L2 achievement. These results provided evidence revealing the directional relationship between L2 self-concept and L2 self-efficacy and evidence supporting the relatively more important role of L2 self-concept in predicting L2 achievement.