A search method of a roaming service for registering a mobile station to a network service includes searching for a home public land mobile network (HPLMN) when the mobile station is first switched on or recovering from lack of service, or sorting an available public land mobile network (PLMN) list, or using a scaling method to obtain a temporary timer period for a periodic search. Sorting the available PLMN list includes sorting according to a location-based order of preferred PLMNs, or sorting according to a data transfer speed supported by each PLMN. The scaling method includes examining a change ratio in the available PLMN list, or a data transfer speed of the RPLMN, or a status of Mobile Country Code (MCC) in order to obtain the temporary timer period.