The paper presents the results of numerical investigation of a natural convective airflow and its heat transfer forming around a single horizontal finned tube (the ratio of a fin diameter to the base tube one is 2.15). Calculations have been carried out at a Rayleigh number Ra = 2×107 or less. Variation of the Ra value was achieved by changing the temperature difference between the heated tube surface and the ambient air, as well as by modifying the geometric scale of the problem. The fin spacing-to-the base tube diameter ratio was varied in the range from 0.09 to 0.55. The optimal fin spacing of 0.23 was determined, corresponding to the maximum heat removal from the taken tube. Good agreement was shown between the calculated results and available experimental data from the literature.