This study aimed to find out the brain hemisphericity and mathematics achievement of high school students. The respondents of the study were the 168 first year high school students of Colegio de San Jose, during school year 2010-2011who were chosen through stratified random sampling. The descriptive and interview methods of research were used in this study. To determine the students’ brain hemisphericity, a Hemisphere Dominance Questionnaire was employed, while the Mathematics Achievement Test was utilized to find out their mathematics achievement. IQ results were taken from the guidance center and interviews were conducted. Frequency count, mean, and standard deviation were the descriptive statistics utilized while Chi-square and ANOVA were the inferential statistics employed. The results revealed that whether taken as an entire group or classified according to gender and intelligence quotient, the dominant brain hemisphericity of the students was the left brain. Their mathematics achievement when taken as an entire group and classified as to gender, intelligence quotient, and brain hemisphericity was “average”. No significant difference existed in the dominant brain hemisphericity of students when classified as to gender and intelligence quotient. Similarly, no significant difference existed in their mathematics achievement when classified according to brain hemisphericity. In solving mathematical problems, the left- brain students tend to write an equation and make logical explanation while the right-brain students tend to make diagrams and illustrations. It was found out in the interview that students tried their best to answer the given problem; however, only very few completed the solution due to several difficulties they encountered.