The mixing characteristic of a chemical reactor can be studied through the resident time distribution (RTD) of the reactor. One major application of the RTD is to perform troubleshooting for a reactor in operation. The deviations from ideal mixing condition such as presence of stagnant region, bypassing of fluids in the system, blockage or failure of the devices in the reactor can be observed through its RTD. In this paper, the study of RTD in a lab scale stirred tank reactor using radiotracer technique is presented. A small scale stirred tank reactor is used to represent a reactor in industry. The objective of this work is to gain experience on the handling of RTD measurement using radiotracer as well as performing RTD analysis of stirred tank reactor with different flow conditions. Tc-99m solution is selected to use in this work due to its chemical property, its suitable half-life and its on-site availability. NaI detectors were placed along the pipelines in order to determine the RTD of the system. The RTD and the Mean Residence Time (MRT) of the reactor is calculated and analysed for the case of normal flow condition and the system with some blockages.