A method and system for providing a data service in interworking wireless public and private networks, allows for data service data being transmitted through the private network when a data service is to be transmitted through the private network within the network where the wireless public network and the wired/wireless private network are interworked. A method of providing a data service in interworking wireless public and private networks, includes transmitting, by a user, a data service request signal; determining, by the private network, based on identifier information through which of the private and public networks to transmit the data service; transmitting a private network data service call to a private content server when it is determined based on the identifier information that it is the private network data service call; and transmitting a public network data service call to a public network content server when it is determined based on the identifier information that it is the public network data service call. Consequently, there is an advantage that a packet data service can be forwarded through any one selected from the public network and the private network. Further, there is another advantage that a data service is available through the private network by modifying the system without changing each user terminal, and thus an additional cost is hardly caused.