X-ray diffraction measurements, vibrating-reed technique and tensile tests have been employed to investigate the elastic properties of single-phase HgBa 2 Ca 2 Cu 3 O 8 + δ . The discontinuities of both Young's sound velocity and linear thermal expansion coefficients for the a and c directions have been observed near the critical temperature of the sample Hg-1223, T c 133 K. Internal friction peaks are found at two other temperature values of 150 K and 250 K. Ferroelastic loops are also observed around 150 K. A thermodynamics analysis of the experimental data is presented. The discontinuity in Young's sound velocity at T c and Gruneisen parameter γ are estimated, which seem to be in agreement with the experimental data.