The basic mechanical properties of antiperovskite manganese nitride, Mn3Cu1-xGexN, known as an isotropic negative thermal expansion (NTE), have been evaluated. The mother material, Mn3CuN, was a soft and ductile intermetallic compound with a positive linear thermal expansion coefficient (alpha=+20 x 10-6 K-1) at room temperature. The thermal expansion properties of Mn3CuN were dramatically changed by Ge doping, resulting in an extremely large NTE material (alpha=-12.5 x 10-6 K-1) by 50% substitution of Cu with Ge. Associate with this, the mechanical properties were found to be strongly modified by Ge doping, and both the stiffness and the hardness of Mn3Cu1-xGexN increased with increasing the Ge content, x. The NTE material, Mn3Cu0.5Ge0.5N, had an enhanced hardness of 570-620 H-v and an exceptionally large Young's modulus of 211-320 GPa. The giant, isotropic NTE compatible with the enhanced value of Young's modulus enables us to fabricate extremely stiff, stable, and lightweight zero thermal expansion composites for the use in many fields of industries.