Nickel-titanium (Ni-Ti) alloys have exotic mechanical and electromechanical properties such as shape memory effect, superelasticity, and electroplasticity. Resembling the deformation behavior of biological materials, the excellent mechanical properties of Ni-Ti alloys make it useful for medical devices including orthopedic implants, orthodontic archwires, and various interventional devices. Ni-Ti alloys also have the potential for applications in microelectromechanical systems (MEMS). With the advance of Ni-Ti thin-film manufacturing technologies and nanoscale characterization techniques, the micromechanical properties have been rigorously studied in recent years. We investigate the carrier dynamics in nickel-titanium (Ni-Ti) alloy thin films using THz transmission spectroscopy. Analyzing the power transmission data and the transmitted waveforms, we obtained the alloy resistivity as a function of Ti concentration. Sharp changes in the resistivity were observed at the Ti fractions of 22%, 44% and 62%, indicating that structural disorder is greatly enhanced when the alloy undergoes a phase transition.