Locked mode instability in the HL-1M tokamak has been investigated in this paper. In the HL-1M tokamak the mode lock instability often appears in with low density discharges (<img src="http:/img/fbpe/bjp/v32n1/27eq01.gif" align="absMiddle"> < 1 × 10(19) m-3). The precursor perturbations on both soft-X rays and Mirnov signals simultaneously increase before mode locking. Their frequencies are the same and gradually decease, which indicate that the perturbation is global. Their existence time-scale is about 10ms. The plasma will suddenly stop rotating when the fluctuation amplitude or frequency reaches a threshold. Mode lock instability can often cause major disruptions. The rotation frequency is decreased to about 3kHz before mode locking in the HL-1M tokamak, which is consistent with the theoretical model.
