A transmission mechanism for spindle head of five-axis machine tool comprises three axes which are driven simultaneously. The transmission mechanism comprises a spindle transmission unit, a B-axis transmission unit and a C-axis transmission unit, and each transmission unit has a motor such that the rotary speed of the spindle is controlled by the two-speed change gear at top side thereof. The cutting unit below the spindle is driven by the spindle for processing. The transmission mechanism has knife replacing unit and blower to blow off dust when replacing knife. The B-axis transmission unit provides 115 degree swing through a gear set and a worm wheel. The C-axis transmission unit provides the dividing head with 360 degree rotation. Therefore, the B-axis transmission unit and the C-axis transmission unit provide the spindle with omni-directional rotation to process irregular curved surface.