A method for monitoring transactions by analyzing packets in a packet processing system is provided. The method includes steps of: (a) an application performance monitoring (APM) device classifying at least one packet, if being acquired in the packet processing system through port mirroring, based on at least either of a port number and an IP address of a client device; (b) the APM device identifying at least one pair of (i) at least one packet transmitted from at least one specific device among the client device and at least one processing device in the packet processing system and (ii) at least one packet transmitted to the specific device in response to the at least one packet transmitted from the specific device by considering at least one packet under identical classification; and (c) the APM device acquiring information on the transactions by using information on the identified pair of the packets.
