The epoxy primer coatings with various size of treated glass fiber was successfully developed using hand brushing technique on carbon steel plate. The glass fibers used were treated with silane coupling agent, 3-aminopropyl triethoxysilane. The Fourier Transform-Infared Spectroscopy (FTIR) affirmed the presence of silanol group at 1221.12cm-1 .Hence, the primer coating was prepared with three different sizes of treated glass fiber (45, 150, and 250 μm) and the corrosive properties has been studied. The corrosion rate was determined using Tafel plot and the immersion test was done in 3.5% NaCl and sea water for 9 days. Formulation of epoxy resin-GF-45μm displayed the optimum size to reduce the corrosion rate of epoxy primer coatings at 8.3713 mm/year and polarization resistance was the highest at 107.48Ω which indicates that epoxy coating have higher resistance towards corrosion, coating adherence and there was formation of more protective film on steel.Whilst immersion in 3.5% NaCl solution and sea water followed the same trend with 45 μm exhibited the less corrosive behavior for 9 days exposure. When the solution penetrated into the coating, the ultra-short glass fibers prevent the water from permeating and prolong the water diffusion path. Thus, it provided good anti-corrosive properties for formulation 2 at 45μm GF. In conclusion, the size of GF plays an important role in determining the corrosive behavior of epoxy primer coatings. The optimum size of GF at 45 μm proved to diminish the corrosiveness of carbon steel plate.