Anisotropic magnetoresistance (AMR) is well known magnetic phenomenon. Despite of its long history, a microscopic origin of AMR has not been fully understood due to lack of experimental technique. Here we exploit terahertz time domain spectroscopy (THz - TDS) to reveal the microscopic origin of AMR. Angle dependent resistivity and scattering time data shows that high resolution and stable THz - TDS is needed to resolve scattering time difference of AMR at room temperature.