The disclosure provides a probe and a method for calculating statistic data of traffic flows. The probe comprises at least one link processor (LP) and a correlation processor (CP). Each LP includes two buffers, receives packets from directional traffic flows, generates information of bi-directional traffic flows based on the received packets, stores the generated information in one buffer within a reporting period and, reports the stored information to CP when the reporting period boundary is reached. The information of each bi-directional traffic flow includes the relevant identification information and statistic data. The CP calculates statistic data of a particular group of traffic flows with a predetermined characteristic based on the reported information, and the other buffer stores information of bi-directional traffic flows to be generated within a next reporting period and the stored information is to be reported to the correlation processor when the next reporting period boundary is reached.