This paper presents a useful knowledge of ejector working principles and its applications in refrigeration technologies. Two-phase flow ejector is used as an expansion device in vapour compression refrigeration systems is one of the efficient ways to enhance its performance .The expansion process (isenthalpic process) represents one of the most important sources of irreversibility in the conventional vapour compression refrigeration systems. Use of a two-phase flow ejector instead of the conventional expansion device is one of the efficient techniques to improve the system performance not only by recover the expansion process loss by generating isentropic expansion process but also by increase the system cooling capacity and decrease the compressor power. Most recent investigations have been concentrated on transcritical ejector-expansion refrigeration cycles (EECs) because of the large improvement potential owing to the high throttling loss. Subcritical EECs have received much less attention because these cycles offer relatively lower improvement potential through the ejector action.