In the present paper, it is reported that a large production of runaway electrons has been observed during the flattop phase of electron cyclotron resonance heating ͑ECRH͒ discharges and in the presence of supersonic molecular beam injection ͑SMBI͒ in the HuanLiuqi-2A ͑commonly referred to as HL-2A͒ ͓Q. W. Yang, Nucl. Fusion 47, S635 ͑2007͔͒ tokamak. For the set of discharges carried out in the present experiment, the ranges of ECRH power and plasma electron density are 0.8-1.0 MW and ͑3.0-4.0͒ ϫ 10 19 m −3 , respectively. A large number of superthermal electrons are produced through the avalanche effect ͓A. Lazaros, Phys. Plasmas 8, 1263 ͑2001͔͒ during ECRH. The loop voltage increase due to SMBI gives rise to a decline in the critical runaway energy, which leads to that many superthermal electrons could be converted into runaway region. Therefore, this phenomenon may come from the synergetic effects of ECRH and SMBI. That is, the superthermal electrons created by ECRH are accelerated into runaway regime via the Dreicer process which is triggered by SMBI. The experimental results are in well agreement with the calculational ones based on the superthermal electron avalanche effect and the Dreicer runaway theory.