Previous studies comparing native English speaking (NES) and non-native English speaking teachers have stressed how each group can contribute their respective language advantages to language teaching and assessment. However, in an interpreting classroom where both groups know each other's native language, little is known about the different assessment behaviour each group could demonstrate, even less about the role their social/cultural background could play in their assessment. To close this gap, this paper studied how NES teachers and native Chinese speaking (NCS) teachers applied their social/cultural knowledge when assessing the same students performing Chinese-English interpreting. By using data triangulation of video recording, assessment sheets and interviews, it identified four types of assessment behaviour: underlining, notating, prior marking and post hoc marking. In particular, NCS teachers were found to have employed more notating yet NES teachers used post hoc marking more frequently. The study further revealed that such divergent assessment behaviour was influenced by the teachers' social/cultural background including their pedagogical beliefs, institution expectations and cultural sensitivity. The paper thus suggested that assessment as part of teaching process should take into account teacher/assessors' social/cultural factors, and the ways their unique background knowledge influenced their judgements.