The interlaminar shear strength characteristics of unidirectional E-glass fiber/epoxy and vinyl ester composites were investigated. The short-beam flexure test (ASTM D-2344) was utilized for comparing the performance characteristics of the two systems. It was found that E-glass fiber/epoxy composite has higher interlaminar shear strength compared to the vinyl ester composite in dry condition. However, for short-term hygrothermal condition, the retained shear strength values were nearly the same. Optical photomicrography revealed that interlaminar shear damage primarily consisted of fiber-matrix interfacial debonding and matrix cracking. Voids were found to have adverse effect on interlaminar shear strength. The influence of voids was studied in relation to mode II crack or damage propagation.