The teaching and learning of mathematics in schools has drawn tremendous attention since the education reform in Taiwan. In addition to assessing cognitive abilities, Taiwan Assessment of Student Achievement in Mathematics (TASA-MAT) collects background information to help depict average student achievement in schools in an educational context. The purpose of this study was to investigate the relationships between student achievement in mathematics and student background characteristics. The data for this study was derived from the sample for the 2005 TASA-MAT Sixth-Grade Main Survey in Taiwan. The average age of the sixth-grade students in Taiwan is 11 years old, as was the sample for the 2005 TASA-MAT. Student socioeconomic status (SES) and student learning-goal orientation were specified as predictor variables of student performance in mathematics. The results indicate that the better performance in mathematics tended to be associated with a higher SES and stronger mastery goal orientation. The SES factor accounted for 4.98% of the variance, and student learning-goal orientation accounted for an additional 10.61% of the variance. The major implication obtained from this study was that goal orientation was much more significant than SES in predicting student performance in mathematics. In addition, the Rasch model treatment of the ordinal response-category data is a novel approach to scoring the goal-orientation items, with the corresponding results in this study being satisfactory.