A monitoring system is proposed in order to detect the condition changes at abrasive/workpiece interface in abrasive cutoff operation. The system can detect the condition changes from the electric power change consumed by the driving motor of lapping tool. The electric power is strongly related with the abrasive effectiveness. When the abrasive effectiveness has been kept a normal state, the electric power also has been kept a constant value. The bodily removal of abrasive leads to reduce the electric power monotonously. The piling up of cutting chips leads to a peculiar electric power change; the electric power increased gradually at the beginning and then began to fluctuate periodically. The severe adhesion of cutting chips to the cutting edges leads to the loss of nominal function in the cutting edges and the electric power input was thus descended suddenly at an early stage of lapping.