Abstract: A method of power transmission in a mechanical/hydraulic type transmission system, by which changeover is smoothly effected between hydraulic power transmission and mechanical power transmission is provided. To this end, there are provided a hydraulic power transmitting means, for transmitting a mechanical power (Pm), which has been converted by a hydraulic motor (42), to the outside (300) by the engagement of a clutch (A), and a mechanical power transmitting means for transmitting a mechanical power (Po) to the outside (300) by the engagement of a clutch (B). Upon changeover from hydraulic power transmission to mechanical power transmission, the releasing of the clutch (A) is performed after the engagement of the clutch (B) is started and before the engagement of the clutch (B) is completed.