The effect of cold plastic deformation by upsetting of the carbon-containing Invar Fe-30.9 % Ni-1.23 % C alloy on structure, magnetic and elastic properties was studied by means of X-ray diffraction and Mössbauer analysis, magnetic and ultrasonic methods. The crystal lattice distortion and additional dissolution of graphite in austenite increase magnetization, hyperfine magnetic fields at iron nuclei, the Curie temperature essentially smearing this phase transition. These changes are caused by weakened interatomic bond in the Invar alloy detected in decreased elastic moduli and the Debye temperature.