The necessity for fabrication detectors with large dimensions, optimal efficiency, high resolution and low price has led to extensive research in the manufacturing of plastic scintillators in the last two decades. The optimization of this type of scintillator is one of the current research topics in the world. In this study, styrene monomer, diphenyloxazole (PPO) as the primary fluorophore and phenyl oxazole benzene (POPOP) as the secondary fluorophore and naphthalene were used to fabricate the plastic scintillators. The samples were fabricated through thermal polymerization without an initiator. The results of gamma spectroscopy using 137Cs and 60Co sources have been shown the increase of the light yield by 5.7% and 2.7% respectively by adding naphthalene in construction of the plastic scintillator. The neutron-gamma discrimination was determined with an analogous discrimination circuit based on the zero-crossing method with a 252Cf source. The results showed that for the sample containing 15 wt% PPO without naphthalene, the FOM and peak-to-valley ratio at a bias of 1000 keVee were 0.91 and 4.18, respectively, while for the sample with 15 wt% PPO and 4 wt% naphthalene, 1.05 and 8.71 were obtained, respectively.