ASTM D2344, referred to as the short beam strength (SBS) method, is a commonly used test method for assessing the interlaminar shear strength of composite materials. The method utilizes a short unidirectionally reinforced composite beam specimen loaded in three-point bending. Due to the three-point loading configuration, the failure mode can be influenced by a combination of interlaminar shear at the midplane of the specimen, transverse contact stress under the central loading roller, and fiber-direction compression stress under the central roller. The objective of the current investigation is to explore two approaches for improving the interlaminar shear strength of carbon/epoxy composites-nanosilica or and/or core-shell-rubber (CSR) particles added to the epoxy matrix- and to explore the use of short beam testing a means of measuring interlaminar shear strength in carbon fiber composites made with these matrix materials. Matrix properties such as mass density, Young's modulus and Mode I fracture toughness are provided along with the constituent contents and SBS of the fiber composites. It is shown that the use of different nanoparticles significantly affects the density, Young's modulus, and Mode I fracture toughness of the matrix. Proper interlaminar shear failure modes were observed in the baseline epoxy and NS toughened composite systems, but not in any of the CSR toughened composites including the CSR/NS hybrids. The reduction of matrix modulus caused by CSR appears to promote an indentation/flexure failure before the interlaminar stresses are sufficiently high to fail the specimen by shear.