A communication device for handling mobility from a long-term evolution (LTE) network to a fifth generation (5G) network comprises a storage device storing instructions of transmitting a first LTE Non-Access Stratum (NAS) message to the LTE network; receiving a second LTE NAS message in response to the first LTE NAS message, from the LTE network; and transmitting a message to the 5G network, after determining to communicate with the 5G network instead of the LTE network, wherein the message comprises a slice information, and the slice information is comprised in the first LTE NAS message or in the second LTE NAS message.