Provided is a power transmission shaft to achieve enhancement of a static torsional strength of a smooth portion. The power transmission shaft according to the present invention includes a torque-transmission teeth portion (2) formed on an outer peripheral surface on an end portion side, and a smooth portion (3) formed on the outer peripheral surface adjacent to the torque-transmission teeth portion (2) so as to have a stepwise reduced diameter. A ratio of a minimum diameter (D3min) of the smooth portion (3) to a minimum diameter (D2min) of the torque-transmission teeth portion (2) is set to 0.9 or more and 1.02 or less. Further, the torque-transmission teeth portion (2) and the smooth portion (3) undergo heat treatment so as to from a heat-treated hardened layer (4). The heat-treated hardened layer (4) is formed to increase in depth from a side of the torque transmission teeth portion (2) to a side of the smooth portion (3). At least a minimum diameter portion (3a) of the smooth portion (3) undergoes full hardening for forming the heat-treated hardened layer (4) from a surface to an axis (X).