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The next generation wireless network is expected to be a heterogeneous. The Radio Access Technology (RAT) selection algorithm, including initial RAT selection and vertical handover, is one of the key research areas in Common Radio Resource Management (CRRM). A suitable RAT selection algorithm can maximize the system performance and Quality of Service (QoS) by allocating users to the most suitable RAT in the case of two or more RATs co-located in the same coverage area. This paper evaluates the performance of three RAT selection algorithms, load balancing based using maximum resource consumption, load balancing based using minimum resource consumption and service based algorithms, in terms of traffic distribution, blocking probability, throughput and throughput fairness for a co-located GERAN/UTRAN/WLAN network. Simulation results show that in terms of blocking probability, the service based algorithm is the worst one when the traffic load is high. In terms of data throughput, the load balancing based using maximum resource consumption algorithm performs better than the other two when the traffic load is low. However, the service based algorithm outperforms the other two when the traffic load is high. In terms of throughput fairness, the service based algorithm achieves the best performance.