The concept of cognitive radio networks has been raised in order to improve efficiency in using the spectrum. For this purpose, the dynamic spectrum allocation is performed among unlicensed users. Cooperative communication is considered as an effective way to deal with the effects of channel fading and increase network coverage. In cooperative communication, one or more intermediate nodes which are known as relay help the source node to send information. Even though cooperative communication has advantages such as reducing bit error rates, reducing outage probability etc, like other communication systems experiences some limitations such as power consumption, bandwidth, and maximum data rate. The features of cognitive radio networks and cooperative communication have led researchers to use both technologies together. Different types of cooperative cognitive radio networks can be considered in which cooperation will exist in spectrum sensing or in transmitting information. In recent years, the idea of green communication has made energy efficiency to be considered as an important criterion in communications networks. For this purpose, in this thesis, the problem of optimal resource allocation considering energy efficiency for a cooperative cognitive radio network has been investigated. Here, the proposed cooperative cognitive radio network consists of a primary source, a primary destination and a number of secondary users, in which secondary users use the underlay mode to transmit their data. Using the proposed relay selection criteria, the primary source chooses a relay from the secondary users to cooperate with primary transmissions. Here, the allocation of resources is based on the energy efficiency of the selected relay and is formulated in the form of a constraint optimization problem. To solve such a problem, a search-based heuristic algorithm is proposed. Finally, numerical implementation of the proposed network with the proposed relay selection and resource allocation algorithm is performed and results of selected relay energy efficiency, secondary network energy efficiency and secondary network lifetime are studied and compared with some existing methods. Finally, the proposed network is simulated without considering any theoretical assumption and the simulation results are studied along with those from numerical implementation. keywords : Cognitive radio network, Cooperative communication, Energy efficiency, Resource allocation.