The current study sought to examine the influence of Korean college students' initial motivation on their English as a Foreign Language (EFL) writing development, as well as the relationships among EFL writing performance, self-efficacy, communicative interest, and instrumental interest. A total of 187 Korean college students completed questionnaires at repeated intervals during one semester of an EFL writing course. Structural equation modeling was used to analyze the longitudinal data. The findings showed that the students' performance and self-efficacy developed significantly, whereas their interest decreased over the semester. The results of growth curve modeling with longitudinal data from three time points suggested that the students' motivational orientation significantly predicted their EFL writing performance at the beginning of the semester. However, the influence of initial motivation on the growth rate of EFL writing proficiency, specifically EFL writing performance, was negative. The conclusion of this study is that the contribution of motivational constructs to EFL writing performance depends on time.