The current study explored the effects of English as a foreign language (EFL) learners' proficiency level on English-medium instruction (EMI) in an online tutoring project. Sixteen Taiwanese college students (tutees) collaborated with preservice teachers (tutors) in the United States in an EMI online tutorial project. The online tutor-tutee interactions were examined to determine if the two tutee groups of two respective proficiency levels were equally receptive to EMI online tutoring based on nine indicators. The results show that learners of both levels could generate approximately equal amounts of language-related discussions and utterances. Overall, proficiency level did not impede tutees' ability to notice their linguistic gaps in interlanguage. Moreover, tutees of both proficiency levels benefited from the textual display of online interactions with their tutors during a task-based learning scheme. Among the nine indicators, successful uptake and feedback type were two strong predictors of tutees' subsequent target language (L2) learning in both groups. The findings offer pedagogical values to promotion of EMI policy in globalized higher education.