The two-dimensional vortex method is applied to the analysis of coolant temperature fluctuations caused by turbulent mixing to examine its applicability to the analysis of thermal striping in liquid-metal fast breeder reactors (LMFBRs). A coaxial jet flow having different temperatures streaming into a stagnant pool is simulated with the method. The calculated velocities are compared with the measurements of an air jet flow. Large scale eddy structures in the mixing region, which cause dynamic large temperature fluctuations, are observed in the numerical simulations. This vortex method can analyze temperature fluctuations caused by large eddies, and is shown to be a useful method for analyzing thermal striping phenomena in LMFBRs.