The behavior of a regular TCP is not good to adjust quickly to the new higher available bandwidth of a newly arrived network after a downward vertical handoff In this paper, we compare three TCPs by mathematical and simulation analysis to the downward vertical handoff With mathematical analysis, we compare the link utilization of NewReno TCP, Scalable TCP, and BIC TCP Among them, the performance of BIC TCP shows best, and it gets better than the other two as RTT increases With simulation analysis, we compare the throughput of NewReno TCP and BIC TCP BIC TCP has higher throughput than NewReno TCP, and the performance gain by BIC TCP gets larger with longer RTT BIC TCP shows the best performance to the downward vertical handoff among the three TCPs That is because BIC TCP can increase the congestion window size exponentially to find a new available bandwidth with a connection sustained.