Thermal plumes in wall free space were numerically simulated with the standard k- e model and the results were compared with the basic experiment which had been conducted by Yokoi. The purpose of this study is to confirm the accuracy and reliability of the numerical simulation of thermal plumes from the view point of practical use of simulation for fire safety planning. The flow fields associated with various conditions concerning the computational domain, mesh dividing system and so on were simulated and compared with the Yokoi's formula in order to evaluate the accuracy and reliability of the CFD performance based on the standard k-ε model. The distributions of Cell Reynolds numbers gave us a good criteria for practically reliable simulations.