Cartesian Cell Registration (CCR) is an effective method to reduce computational time for particle search and positioning, for example, in the Direct Simulation Monte Carlo (DSMC) method for dilute fluid flows and multiphase flow calculations. In this chapter, an efficient FORTRAN algorithm for the CCR method is presented to further reduce both the computational time for registration and computer memory. With this algorithm, the computation time for searching the target cell where the target particle exists is reduced by as much as 44,234 times. Moreover, this algorithm was successfully applied to the DSMC simulation, resulting in a 369-fold reduction in computational time compared to the brute-force approach, that is, searching all target cells until the cell in which the target particle is present is found.