To improve the efficacy of extracorporeal shock wave lithotripsy (ESWL) treatment, it is desirable to identify the physical properties of urinary calculi could offer direct correlation with their fragilities during ESWL and thus could be used to guide treatment procedures for more effective stone fragmentation. Thirty stone specimens removed surgically were compressed by an axial testing system to measure the compressive strength and trace the stress-strain curve. Image analysis software SigmaScan (Jandel Co.) was used to calculate the area under the stress-strain curve, the modulus of toughness, for each stone. The values of compressive strength measured were similar to those reported by other researchers. The modulus of toughness of urinary calculi correlates with clinical representation of the stone fragility during ESWL. The modulus of toughness could be an index to evaluate the physical property of urinary calculi that could be used to guide treatment procedures for more effective stone fragmentation.