Understanding motivational beliefs such as expectancy and value that shape students' persistence and decision to pursue a STEM career, obtaining valid and reliable measures for these dimensions, and developing strategies using this data are critically important to ensure students' persistence in the STEM pipeline. Therefore, this study aims to develop a tool to measure middle school students' STEM motivations within the expectancy and value concepts framework. The trial version of the scale was conducted on 967 middle school students in the 5th, 6th, 7th, and 8th grades. The study group was randomly divided into two groups. EFA was conducted on the data obtained from the first sub-group (n=479), and CFA was performed using the data obtained from the second sub-group (n=488). The results of a series of CFA performed to test three different models developed based on the theoretical structure, Model 3, the second-order single-factor structure composed of 5 sub-dimensions was found to be a successful model. This measurement tool would allow determining motivational beliefs within the expectancy-value concept that can be targeted to encourage students' interest in STEM fields, as well as help design interventions for these structure(s), and evaluate the effectiveness of these interventions.