This study aims to validate the instrument of Mathematical Beliefs among secondary school Mathematics teachers. This study is a completely quantitative study that uses survey methods using questionnaire procedures. A total of 100 Mathematics teachers were involved as respondents in this study. The data were analyzed descriptively by access Alpha Cronbach reliability and EFA analysis using SPSS software. The results of the analysis show that Alpha Cronbach value is 0.929 which is more than 0.60. Results from the Exploration Factor Analysis (EFA) show three factors with Eigen values greater than 1.0. The KMO value (Kaiser-Meyer-Olkin) 0.832 >0.6 indicates that the items in the variable of Mathematical Beliefs are sufficient for inter-correlation. While the Bartlett Test was significant (Chi Square 1955.072, p <0.05), an anti-image value (Measure of Sampling Adequancy, MSA) for items correlation exceeded 0.6. However, there are 11 items that need to be removed because the values of factor loading obtained are less than 0.60, items C5,