IThe participants were 124 ninth graders (50 male, 74female) from the Demonstration School Chiang Mai University, Thailand. Most of them were 14-15 years old. The questionnaire included the Attitudes toward Mathematics Inventory based on Wade (2014) and The Indiana Mathematics Beliefs Scale based on Kloosterman & Stage (1992). The study was conducted in the second semester of 2015 academic year by administering the questionnaire to all participants from 3 classes of ninth grade students that were randomly selected during one class period. The questionnaire was designed to take 30 minutes. Then, 12 students (6 males, 6 females) with mixed mathematics achievement (4 high, 4 average, and 4 low achievers) were selected to be interviewed about their attitudes and beliefs about mathematics. In data analysis, the data from the attitudes and beliefs scales were used to computed means and standard deviations for each scale. Theses statistical methods were applied to the sample by focusing on all students. Gender differences overall were set for statistical significance using two sample t-tests. Mathematics achievement differences (high, average, and low) were tested for statistical significance using analysis of variance (ANOVA). The data from interviews were analyzed by descriptive analysis.