It is well known that successful second language (L2) learners are motivated individuals. Accordingly, L2 researchers have tested the predictive power of different motivational constructs on language learning outcomes. However, this perspective appears to neglect the assessment of achievement as a predictor of future motivation. To assess this possibility, we first employed the latent growth curve model (LGCM) to evaluate the initial values and growth rates of the two variables. We further applied a newly developed statistical method, the random-intercept cross-lagged panel model (RI-CLPM), to study the causal relationship. A total of 226 language students were monitored for 17 weeks at three time points. The analysis showed an increasing trend in the association between the growth levels of both variables. However, students' autonomous motivation at Time 1 appears to affect achievement at Time 2. Further, the second wave of the RI-CLPM illustrated that achievement at Time 2 impacted autonomous motivation at Time 3, while motivation failed to predict scores on achievement at Time 3.