Abstract: In this paper, the turbulent heat transfer in the horizontal helical coil is investigated in detail in a wide range of experimental parameters. The experiment shows that the enhancement of heat transfer in the coil results from the effect of turbulent flow and secondary flow. If the Reynolds number becomes larger, the secondary flow gives less contribution to the enhancement heat transfer, and then the average heat transfer coefficient of the coil is closer to that in straight tubes under the same condition. The cocal heat transfer coefficients on the cross section of the coil are not uniformly-distributed. The cocal heat transfer coefficient on the outside is three or four times period which is half of the average heat transfer coefficient. A correlation is proposed to calculate the heat transfer coefficient distribution on the cross section. The average heat transfer coefficients on the cross section change period along the direction of the fluid flow.