Electronic Apex Locator (EAL) has been developed to determine the true apical foramen location and working length of root canals in endodontic treatment process. It has been long stories of EAL devices since the principle idea was first reported. Since then, many modified ideas and varieties commercial products of electronic apex locators have been studied and introduced in to markets. However, the prices of commercial EAL are mostly expensive which limit the clinical usages in the under-developing or developing countries. From this economic reason, this project was purposed to study on the design and construction of electronic apex locator which has the low-cost price. The design and construction of this device based on the principle of clinical utilization, microcontroller and electronics knowledge. The In Vitro test, on functions of the prototype compared to the commercial device, was performed in 20 root canals which embedded in artificial oral condition. We found that the percentage error of our product compared to the commercial device was 1.74%. It showed that the prototype of EAL can correctly identify the apex location and calculate the working length for endodontic treatments with has cheaper price than that of the commercial products and meets the main purpose of project.