The aim of the study was to examine the relationships between gender, previous knowledge, different personality traits, subject-specific motivational dimensions and students' math grade in secondary school. A total of 386 first-year students (142 boys and 244 girls) from secondary schools in Slovenia (mean age was 15.7 years) participated in the study. Different measures were used to assess students' previous knowledge, personality traits, subject-specific interest, self-efficacy, immediate action and procrastination/distractibility. Path analysis was used to test the model of direct effects of gender, previous knowledge and personality traits on math grade and indirect effects of previous knowledge and personality traits on the grade through subject-specific motivational variables. The results of path analysis revealed that we can explain 40% of the variance in math grade with variables included in the research. Gender, previous knowledge and personality traits have direct and indirect impacts on achievement. Among personality variables, conscientiousness proved to be the most important direct and indirect predictor of math grade. A significant part of variance in math grade can be explained by taking into account more general individual differences on the one hand and subject-specific motivational processes, which act as mediators, on the other hand. In promoting students' math achievement, teachers should take into account students' personality traits and try to develop their motivational self-regulation.