In addition, to refer with the changes in intermolecular structure of vinyl ester resin of why the mechanical properties improved at the optimal amount addition of MEPECO at 5%, FTIR was implied. This reduced FTIR intensity may indicate that the MEPECO is reacting with vinyl ester resin assuming a formation of new cross-linking and chemical interactions. This also confirmed the improved adhesion which VE_5%MEPECO previously showed in wettability property, that main caused the mechanical properties improvement in terms of flexural strength and decrease in flexural modulus. The possible homogeneity and dispersion of 15%MEPECO over vinyl ester resin also might be the reason of the overall changes of VE_neat FTIR intensities.