This study focused on the relationships between teacher characteristics and students' mathematics achievement in EU countries, including Hungary, Italy, Lithuania, Malta, Slovenia, Sweden, and Turkey, which are the only EU countries participated in TIMSS 2015 at the eighth-grade level. The data consisted of the sample of 31,969 eighth-grade students and their teachers (n = 1687) in TIMSS 2015. The "Qualities of Effective Teachers" were used as a conceptual framework for variable selection and 24 teacher characteristics were tested across these EU countries through Hierarchical Linear Modeling technique (HLM). As a result, 17 characteristics suggested as effective by previous researchers were found to have significant relationships with students' mathematics achievement in the pooled sample while there were discrepancies when investigating countries individually. The characteristics found to have non-significant, or negatively-significant relationships as well as the discrepancies in the individual countries raised concerns regarding the implementation of these characteristics in the classroom environments, along with the scope of the study.