This study examined personality similarity between teachers and their students and its impact on teacher judgement of student achievement in the domains of reading comprehension and mathematics. Personality similarity was quantified through intraclass correlations between personality characteristics of 409 dyads of German teachers and their students. This similarity index was combined with teachers' global and task-specific judgements of student achievement. Personality similarity has a significant effect on global judgement in both domains under study. Students who are similar to their teacher are judged more positively than students who are dissimilar, even when students' test performance is controlled. This effect could not be verified for task-specific judgements. Results indicate that impact of potential sympathy bias in social judgements differs between different types of judgement. That is, global judgements are more likely to be biased than more specific judgements. Theoretical and educational relevance of the findings are discussed.