The present invention is concerned with a method of thermally decomposing a hydrocarbon and a thermal decomposition tube used therefor. A plurality of spiral fins are formed on the inner wall surface of the thermal decomposition tube. Since no-fin portions are formed on portions of the inner wall surface, the flow of a hydrocarbon through the no-fin portions is made turbulent to a suitable degree. For this reason, coke is prevented from adhering to the inner wall surface of the thermal decomposition tube.