Tensile properties and morphology of tetrafunctional multigraft copolymers are investigated dependent on PS volume fraction and number of branch points. It is found that tetrafunctional multigraft copolymers with 22 vol % PS and seven branch points show a surprising high strain at break of about 2100%, about double that of commercial thermoplastic elastomers (TPE's) such as Kraton. With increasing number of branch points, strain at break and tensile strength increases, and strain at break is about 2300% for a sample with 10 branch points. Investigation of morphology using transmission electron microscopy indicates that a sample with 22 vol % PS has a wormlike microphase-separated structure with much lower long range order than other TPE's such as Kraton. The multigraft copolymers of this study have two PS arms at each branch point. This, together with a large number of branch points per molecule, allows the elastic PI backbone to couple into a large number of reinforcing PS domains, resulting in huge elasticity, combined with a high tensile strength. Two parameters for adjusting mechanical properties of multigraft copolymers are found in our investigations: (1) functionality of the graft copolymer, tri-or tetrafunctional, and (2) number of branch points per molecule.