This study examined naturally-occurring university classroom interactions at Iranian universities and provided an analysis of silence patterns as politeness strategies used by male and female students. Since empirical studies of silence in classroom settings are scarce, this paper aimed to explain such phenomena using participant interviews, classroom observation and detailed discourse analysis of classroom interaction. Silence patterns and their interpretations were scrutinized in these observations and were discussed in relation to specific conceptualization of politeness and devices employed to exercise it. The study found that females seem to be the most silent in the cross-sex classrooms, while the distribution of silence is more nearly equal in the same-sex classrooms. Based on the comments from follow-up interviews, reasons for intentional silence as a politeness strategy were categorized into four groups: silence as a face-saving strategy, silence as a ‘don’t do the FTA’ strategy, silence as a power strategy, and silence as an off-record strategy.