In 3D electronic package technologies, through silicon via (TSV) plays a critical important role. TSVs are usually fully filled by electroplating copper, namely TSV-Cu, which has very different mechanical properties from bulk copper. To obtain the mechanical properties of the TSV-Cu, the Berkovich nanoindentation tests were conducted, and the Oliver-Pharr algorithm and the continuous stiffness measurement method were used to acquire the elastic modulus and hardness. Then finite element modeling (FEM) simulations are adopted for reverse analysis of the nanoindentation loading process to determine the representative stress and strain of the TSV-Cu by comparing the maximum value of simulated load to that of experimental load. The strain hardening exponent of the TSV-Cu is determined by dimension functions. The yield strength of the TSV-Cu is acquired by substituting the representative stress, the representative strain and the strain hardening exponent into a power law stress-strain constitution. Finally, a power law elastic-plastic stress-strain relationship of TSV-Cu is built. The obtained elastic modulus and hardness of the TSV-Cu are 155.47 GPa and 2.47 GPa, respectively; the strain hardening exponent is 0.4892 and the yield strength is 47.91 MPa.