The triangular routing is one of the main problem in MIPv4. It leads to many attendant problems, namely the increment of impact of possible network partitions, load on the network and delay in delivering packets. The next generation IPv6 is designed to overcome this kind of problems. However, IPv4 and IPv6 are anticipated to coexist for a period of time. In addition, during IPv4 to IPv6 transition period, when Network Address Translation Protocol Translation (NAT-PT) is used to connect IPv4 and IPv6 networks together, the triangular routing cannot be avoided. In this research, we study the effect of triangular routing and Mobile IP-Application Layer Gateway (MIP-ALG) in mixed IPv4/IPv6 networks using NS-2 simulator. Three traffic models, namely HTTP, FTP and VoIP, are used in our simulation. The performance evaluation criteria are throughput and mean end-to-end delay. The simulation results show that the triangular routing degrades the performance of the mixed IPv4/IPv6 network. It is obvious that triangular routing reduces the peak throughput of HTTP and FTP traffics while it does not affect peak throughput of VoIP traffic. The mean end-to-end delay for HTTP, FTP and VoIP increase 45.45%, 47.61% and 52.74%, respectively. Finally, it is found that MIP-ALG can reduce the triangular routing effect, when direction of transmitting data is from IPv6 to MIPv4 network.