This paper examines the effect of the fiber content and fiber length on tensile, fracture and thermal properties of syntactic foam. Results showed that a hybrid structure demonstrates a significant increase in the ultimate tensile strength, sigma(uts), and Young's modulus, E, with increasing fiber loading. Interestingly, the fracture toughness, K-Ic., and energy release rate, G(Ic), increased by 95% and 90%, respectively, upon introduction of 3 wt% short carbon fibers in syntactic foam, indicating the potent toughening potential for short carbon fibers in syntactic foam systems. SEM and OM studies identified the presence of several toughening mechanisms. An estimate of the contribution from each toughening mechanism by composite theory and fractography revealed that the specific energy required to create new surfaces was enhanced by the presence of fibers and was the main contributor to the toughness of the short fiber reinforced syntactic foam. (c) 2007 Elsevier Ltd. All rights reserved.