To generate high-quality research, a reliable instrument is required. This study aims to develop an instrument that can measure mathematical dimensions of elementary school students appropriately, for use in mathematics learning on spatial geometry. Each statement item in Mathematical Anxiety Instrument was developed based on 3 learning aspects, namely Attitudinal, Cognitive, Somatic. This study uses Research and Development (R&D) method with a quasi-experimental design on geometry topic. The sample is 100 sixth grade students in an elementary school located in Karawang Regency. The instrument developed in this study is in the form of a questionnaire with closed-ended questions. The validity test analysis implies that 23 out of 30 items on the instrument are valid. This data is supported by Cronbach's Alpha test results, where all item has reliability value higher than 0.80 indicating that the instrument being tested has a very high-test reliability. Therefore, 23 items out of 30 items were developed into instruments for measuring mathematics anxiety that applicable in Mathematics learning activities. Thus, it leads to the conclusion that this mathematics anxiety questionnaire can be utilized as an instrument for future research.