The present invention relates to a vertical handoff method. According to the present invention, a serving network of a mobile station is determined. Herein, the mobile station that can access a first network having relatively wide service coverage and a second network having relatively narrow service coverage in an area where service coverages of heterogeneous networks are overlapped. When accessing the serving network of the first network, it is determined whether a received signal strength received from the second network is greater than a first threshold value. In addition, a first vertical handoff is decided by using a result of prediction of a transition pattern of the received signal strength that is greater than the first threshold value. When the serving network is the second network, it is determined whether the received signal strength received from the second network is less than a second threshold value. By using a result of prediction of a transition pattern of the received signal strength that is less than the second threshold value, a second vertical handoff is decided. Herein, the first threshold value is greater than the second threshold value.