Together with the growth of computer network activities, the growing rate of network attacks including hacker, cracker, and criminal enterprises have been advancing, which impact to the availability, confidentiality, and integrity of critical information data. In this paper, we propose a RealTime Intrusion Detection System (RT-IDS) using Decision tree technique to classify an online network data that is preprocessed to have only 13 features. The number of features affects to the RT-IDS detection speed and resource consumption. In addition our RT-IDS can classify normal network activities and main attack types consisting of Probe and Denial of Service (DoS). Hence, it helps to decrease time to diagnose and defense each network attack. The results show that our RT-IDS technique offers the detection rate higher than 98%, while consuming less than 25% of CPU and 94.5 MB of memory on full traffic load of 100 Mbps.