Nowadays, due to the threats of radioactive materials, control of borders AND sensitive facilities is of particular importance. For this reason, every year the International Atomic Energy Agency publishes a report on the theft, loss or lack of monitoring AND control on the movement of the radioactive materials in some countries. Therefore, it will be necessary to create AND introduce new methods to deal with such threats. One of the most effective systems for detecting radioactive materials AND contaminants are radioactive portal monitors. In this study, the results of performance evaluation of large plastic SCINTILLATION DETECTORS for use in this type of monitoring systems are presented. Initially, the design AND construction of electronic circuits AND mechanical holders were done. Then, the functional tests of the device were performed using 60Co AND 137Cs sources. Finally, the minimum detectable activity using this monitoring device was determined. The results of the tests to determine the minimum detectable activity of the system showed that it can detect a 60Co source with a minimum activity of 1 μ, Ci at a maximum distance of 100 cm AND a 137Cs source with a minimum activity of 2 μ, Ci at a maximum distance of 75 cm respectively. Regard to the obtained results, it was found that the developed monitoring system has the ability to detect the radioactive sources with good accuracy.