The investigations of the oscillation-starting characteristics of a ladder-type RF circuit are proposed to overcome the limitation of high ohmic loss for development of millimeter-wave extended interaction oscillators （EIOs）. Based on PIC-simulations， quantitative calculations and theoretical analyses， the designed and fabricated W-band EIO is proved to have the possibility of greatly reducing the oscillation-starting current. By optimizing five aspects including the gap number， cavity dimension， field distribution， operation voltage， and surface loss， the oscillation-starting current of the EIO can be reduced to 0.43 A with a beam voltage of 17.5 kV. According to cold test experiment， the output power attenuation is analyzed and predicted.