Residence time distribution (RTD) is a smart concept in chemical reaction engineering, and has been playing an important role in diagnosis and optimization of chemical reactors. RTD is conventionally determined by experiment by using a tracer to track the motion of reacting fluids. Currently the numerical simulation of RTD also follows the same route. In this work, the numerical method of retrieving RTD directly from the simulated flow field is proposed in view that the RTD itself is the intrinsic property of the fluid flow in a reactor. RTD can be determined by following the streamlines of all entering fluid particles, or by accumulating the residence time increment within all control volumes that a fluid particle traverses in a random way. A numerical case of a radial flow reactor is presented to demonstrate the equivalence of these two methods.