The effects of organically modified montmorillonite clay nanoparticles in neat vinyl ester resin were investigated. Organically modified MMT clay particles, Cloisite10A and Cloisite30B were dispersed into neat organic vinyl ester resin by using a ultrasonic method. Morphology of the dispersed clay particles in the nanocomposites formed during polymerization, was characterized by X-ray diffraction (XRD) spectroscopy and found that intercalated nanocomposites were formed. The thermal properties such as decomposition and glass transition temperatures, percent weight retention and tan δ were determined by thermo gravimetric analyzer (TGA) and dynamic mechanical thermal analyzer (DMTA). A decrease in glass transition temperature was found in both vinylester/Cliosite10A and vinylester/Cliosite30B nanocomposites. The tensile, flexural and impact properties of the nanocomposites were determined by universal testing machine (UTM) and Izod impact tester. Due to the incorporation of MMT nanoparticles in vinyl ester resin an improvement in mechanical properties was observed. The effect of water absorption on nanocomposites was investigated and a significant decrease in water absorption of the vinylester nanocomposites was found.