An experimental study has been conducted on steady-state natural convection heat transfer from vertical helical coil tubes in heat transfer oil of a Prandtl number range of 250-400. Fifteen coils are used in this experiment. These coils are classified into five groups; each group has a specified coil diameter-to-tube diameter ratio for two, five, and ten turns. The helix coil to tube diameter ratio are 30, 20.83, 17.5, 13.33, and 10. The heat transfer data were generated and correlated using coil length as a characteristic length. Correlations are presented to calculate the average Nusselt number in terms of Rayleigh number for each group of coils, and an overall correlation covering all the coils is obtained. Furthermore, two more general correlations covering the current data and those of Ali [1] are obtained, and the results show that the average Nusselt number for oil is higher than that for water at the same Grashof number.