A valvetrain mechanism of an engine, including a drive shaft configured to rotate in synchronization with the engine, a link arm having a large end and a small end, the drive shaft being inserted into and passed through the large end, a valve lift control shaft disposed parallel to the drive shaft and having an eccentric portion, an oscillating arm disposed rotatably at the eccentric portion of the valve lift control shaft and interconnected with the small end of the link arm via a first rotation support point, a link rod interconnected with the oscillating arm via a second rotation support point positioned at a same side as the first rotation support point with respect to the valve lift control shaft, and an oscillating cam interconnected with the link rod via a third rotation support point and operated correspondingly with an operation of the drive shaft to thereby open a valve. A thickness of the small end of the link arm in a direction of the drive shaft is thinner than a thickness of the large end of the link arm in the direction of the drive shaft.