The aim of the present study is to examine the effect of demographic characteristics of students in Turkey upon their performance in TIMSS, an international assessment exam and Secondary Education Transition Examination which is a national exam (OGS). One of the fields of sciences, biology is taken into account as student performance. As a result of the t-test analysis, it was determined that gender has a statistically significant effect on the students' TIMSS and TEOG exam results; besides, it was found that female students are more successful compared to boys Whether students receive private tutoring which refers to the economic characteristics of the students has not statistically significant effect on TIMSS biology scores, while it has a positive and significant impact upon OGS biology scores. Moreover, findings revealed that parental educational level has a positive and significant effect on TIMSS and OGS scores of the students.