Abstract: Plasma disruption on the Tokamak device often generates a lot of runaway electrons. Primary (Dreicer) acceleration and secondary avalanching are two mechanisms responsible for the generation of runaway electrons during disruptions. The behavior of runaway electrons generation was investigated at different phases of discharges on Experiment Advanced Superconducting Tokamak. It is found that primary acceleration is the main mechanism in current ramp-up phase while avalanching played an important role in current quench. In the paper, the diagnostics system is described, and the mechanisms of runaway electrons generation in different phases of discharges are presented by hard X-ray measurement.