The depth of anaesthesia (DOA) is generally considered adequate if the patient neither moves in response to surgical stimulus nor shows any signs of autonomic reflexes. So the measuring of depth of anaesthesia is very important because it helps the anesthetist to monitor the anesthetic state of the patient from the start of giving the anaesthetic drugs up to the patient awakeness and helps him to control the required amounts of the anaesthetic drugs during the surgery. There is no direct method to judge the DOA. It seems logical that, since the brain is the primary site of action of anaesthetic agents, there should be some measure of anaesthetic effects on the brain. The brain electrical activity (Electroencephalogram (EEG) and evoked potentials (EP)) have been studied to develop quantitative monitors of DOA. Due to the large problems with the EEG monitoring, EP monitoring has been suggested as an alternative indicator of anaesthetic depth. The mid latency auditory evoked response (MLAER) was seemed to be the most promising measure of DOA, which is independent of the agent being used. The aim of this paper is to propose an approach to explore a reliable index reflecting the changes in amplitudes and latencies of MLAER waves, which are related to depth of anaesthesia