An electric rotary hairbrush includes a handle unit and a brush unit, the handle unit having a transmission mechanism controlled by a switch to rote the rotary brush unit, a positive and negative pole electrodes respectively mounted in the circular flat top thereof and electrically connected to an inside electric circuit for power output, the rotary hairbrush unit having a rotary cap capped on the handle unit, and an electrically insulative disk mounted inside the rotary cap, the electrically insulative disk having two annular metal contacts concentrically provided at the bottom side and respectively disposed in contact with the positive and negative pole electrodes, and positive pole and negative pole terminals fixedly provided at the top side and respectively electrically connected to the annular metal contacts, and an electric heater mounted in the comb holder and electrically connected to the positive and negative pole terminals.