A method for processing a network packet is used for a routing apparatus. The routing apparatus includes a forwarding engine module and a core module. In the method, a network packet from a source interface is received. Whether or not the network packet meets a Network Address Translation (NAT) packet condition is determined. Subsequently, the following steps are executed when the network packet meets the NAT packet condition. In these steps, transmission information of the network packet is read and a NAT mapping table is searched by using the transmission information. When a mapping entry corresponding to the transmission information is found in the NAT mapping table, the method involves enabling the forwarding engine module to update the header of the network packet according to the NAT forwarding information of the corresponding mapping entry and set a destination interface of the network packet.