The purposes of the study were to investigate the relationship between factors and self-determination in learning mathematics of grade 8 students in Thailand and to study factors affecting self-determination in learning mathematics of grade 8 students. The participants were 500 students in an area called Bueng Kan province in Thailand. They were selected using multi-stage random sampling from the population of 872 grade 8 students in the area. The instrument was an assessment of grade 8 students' self-determination in learning mathematics. The assessment consisted of 70 items assessing factors that could affect students' self-determination and 30 items assessing students' self-determination in learning mathematics. The assessment items were proved with 0.34-0.62 discrimination and 0.96 reliability. The data were analyzed using percentages, mean scores, standard deviation, and Stepwise Multiple Regression Analysis. The variables used in the data analysis were intrinsic motivation (X[subscript 1]), attitude toward mathematics (X[subscript 2]), learning behaviors (X[subscript 3]), teacher instruction technique (X[subscript 4]), teacher-student relationship (X[subscript 5]), problem-solving abilities (X[subscript 6]), and social support (X[subscript 7]) as predictions while self-determination in mathematics learning (Y) was set as the criterion. The results of the study indicate that the Stepwise Multiple Regression Analysis indicates 4 factors including intrinsic motivation, learning behaviors, teacher instruction technique, and the teacher-student relationship showing a correlation to self-determination in learning mathematics. Moreover, the predictable factors were intrinsic motivation (X[subscript 1]), learning behaviors (X[subscript 3]), teacher instruction technique (X[subscript 4]), and teacher-student relationship (X[subscript 5]). They could predict approximately 76.00% of self-determination. The predictable equation in raw scores and standard scores were, Y' = 0.06 + 0.52X[subscript 1] + 0.27X[subscript 3] + 0.09X[subscript 5] + 0.12X[subscript 4]; Z'[subscript s] = 0.53Z[subscript 1] + 0.26Z[subscript 3] + 0.11Z[subscript 5] + 0.13Z[subscript 4].