The present article has reported the effects of several nanofiller's aspect ratio, length and interfacial strength on Mode-I fracture toughness (K IC) of geopolymer as the matrix of continuous fibre reinforced composites. These nanofillers have been chosen based on the variations in the surface chemistry and nature of interfacial bonding with geopolymer, which include Carbon, Alumina and Silicon carbide. Geopolymer matrix was subjected to the addition of single volume fraction, 2% of each type of nanofiller with two aspect ratios, designated as nanoparticles and nanofibers. Notched beam flexure tests (SEVNB) of neat and each nanofiller reinforced samples suggest that, while baseline K IC of neat geopolymer improved with heat treatment, nanofibers with high interfacial bond strength showed maximum capability in further improving K IC. Among those nanofibers, 2 vol% Silicon Carbide Whisker (SCW) showed the largest improvement in K IC of geopolymer, which is similar to 164%. After heat treatment at 650 degrees C, SCW reinforcement was also found to be effective, with only similar to 28% lower than the reinforcing performance at 250 degrees C, while the performance of Alumina Nanofiber reinforced geopolymer notably reduced. SEM and EDS analysis suggested that the inhomogeneity in neat geopolymer and length of nanofibers control the reinforcing capability as well as crack propagation resistance of geopolymer. For instance, minimum length of nanofillers to toughen this geopolymer at 250 degrees C was required as similar to 2 mu m. The results further suggested that the sample failure occurred due to the dominance of tensile failure of nanofibers over the interfacial separation.