While it is not difficult to see why beginner English as a second/foreign language (L2/EFL) learners’ willingness to communicate (WTC) may be inhibited, it remains an intriguing issue why many advanced L2/EFL learners, such as postgraduates, still exhibit low levels of WTC in content classrooms where English is used as a medium of instruction. The present study addressed this gap by exploring what factors contributed to four Chinese postgraduate students’ WTC in English during classes that were lectured in English. Through the qualitative analysis of data gathered from individual interviews, focus group interviews, and class observation field notes, the results showed that individual, environmental, social-cultural, and educational dimensions jointly influenced the students’ WTC in English during class. The findings highlight the value of adopting a situational perspective to understand WTC and call for attention to the difference between WTC in using English for general communication and WTC in English Medium Instruction (EMI)-mediated academic classrooms. This article concludes with implications on how postgraduates’ WTC in English can be better promoted.