The present study aimed to explore English as a foreign language (EFL) learners' first impressions of their language teachers. The participants were 679 male and female Iranian EFL learners from different age groups and English language proficiency levels at five language institutes and four universities. The main data collection instrument was a validated 47-item questionnaire. The findings showed that at the end of the semester, EFL learners' first impressions of their language teacher, formed based on short observations of thin slices of behavior (less than five minutes), did not change significantly. The results also indicated that there were significant differences in the extent of first impression change between male and female participants and different language proficiency levels (low- and high-proficiency). Moreover, there was no statistically significant difference between different age groups (teenagers and adults) in terms of the extent of first impression change. The pedagogical implications of the findings regarding the impacts of language learners' thin-slice judgments of the teachers' success in EFL contexts are discussed.