The nanoindentation fracture of ultra thin amorphous carbon films on silicon by a conical indenter was studied, and compared with the nanoindentation fracture of the same films on silicon by a cube corner indenter. It is found that the conical indenter gives an indentation fracture similar to that by the cube corner indenter. The cube corner indenter results in a larger stress concentration leading to fracture of films at lower loads. Based on the analysis of the energy release in cracking, the fracture toughness of the films was calculated. The results show that the conical indenter gives the same fracture toughness values as the cube corner indenter and these calculated values are in good agreement with those measured by conventional methods.