In this work, we investigate mechanical properties and fracture behaviors of thermally reduced graphene oxide (TRGO)-filled epoxy composites with two different dispersion/ interface levels. The amphiphilic surfactant treatment of TRGO was found to promote the dispersion in both water and epoxy, but also produced improved TRGO/epoxy interface. As expected, incorporation of untreated or treated TRGO sheets into epoxy increases the storage modulus and the glass transition temperature. In particular, significant improvements in both tensile strength and fracture toughness (KIC) of the TRGO/epoxy composites were also obtained after the surfactant treatment processing. For example, at a very low loading of 0.20 wt.-%, the surfactant-treated TRGO enhances the tensile strength and KIC of epoxy by $51 and$65%, respectively; while the corresponding untreated TRGO results in almost unchanged strength and produces $52% improvement in the KIC value. Based on the fractography observation and analysis, several failure mechanisms, e.g., crack bridging, pull-out/debonding, and fracture of TRGO as well as matrix plastic deformation, were identified and correlated to the increased fracture toughness of the two composite systems studied.