The mixing of fluid in a CSTR in presence/absence of impeller and baffles is investigated numerically using Computational fluid dynamics software package, Ansys Fluent. At the inlet of the CSTR, tracer (KCl) is injected by step change and the tracer concentration at the exit is noted with time to determine the age distribution function I(theta). The study helps to understand the residence time distribution (RTD) of CSTR. The CFD simulated predictions are compared with the literature data and a good agreement is found. The mixing performance of CSTR is studied using system parameters like tank Reynolds number and impeller rotation. The mixing characteristics such as Holdback, Segregation, mean residence time, variance and number of ideal CSTR in series equivalent to single actual CSTR are also determined and all these study ensures that the flow behaviour changes from dispersion to ideal mixing with increasing the tank Reynolds number and impeller speed.