Abstract: The small-strain shear modulus plays an important role in the calculations of ground seismic response and deformation in deep excavations. The values of the small strain shear modulus G₀ of Shanghai soils by field and laboratory tests are investigated and compared, and empirical formulas are proposed to predict the values of G₀. The results indicate that G₀ mainly depends on the soil type, void ratio and mean confining pressure. The values of G₀ from field tests are considerately higher than those from laboratory tests. The correlation between the small strain shear moduli from field and laboratory is also proposed. The numerical analysis is also carried out to calculate the deformation in a deep excavation in Shanghai. The small strain hardening model (HSS) is used, together with the measured small strain shear modulus. The values of G₀ are essential for the deformation of the excavation, and the predicted deformation using the G₀ value from field measurement is more accurate than that using the laboratory data based on the measured deformation in the excavation.