The objective is to examine the changes in the effectiveness of motivational strategy use by teachers during one semester in the English as a foreign language (EFL) classrooms. More specifically, we investigate differences in effectiveness changes for each motivational strategy used according to students' English proficiency levels and their original motivational intensity levels in order to understand the process by which instructors influence students' motivation. The participants were an instructor and 222 university students. Two distinct clusters were found within the student sample based on cluster analysis: Cluster A) students with higher English proficiency and higher motivational intensity; and Cluster B) students with lower English proficiency and lower motivational intensity. The results showed that (a) the two clusters did not exhibit a similar trend in terms of the relationship between frequency of motivational strategy use by teachers and students' motivation except with respect to two motivational strategies; (b) seven strategies showed positive correlations at least once in the semester in either Cluster A or Cluster B; (c) five strategies showed no significant correlations throughout the semester; and (d) there were three strategies that did not show a positive correlation; however, these strategies were associated with a negative correlation throughout the semester in either Cluster A or Cluster B.