A chain saw includes a wheel and a chain plate exposed to a saw casing, a chain trained over the wheel and the plate, and a driving device for driving the wheel. The plate is formed with an elongated slot. A chain tension adjustment mechanism includes a shielding cover disposed between the casing and the plate; a slide element disposed movably between the plate and the shielding cover, has a protrusion extending through the slot in the plate; and a biasing member biasing against the slide element and the shielding cover so as to push the slide element away from the wheel, wherein, movement of the plate toward the wheel against biasing action of the biasing member results in retraction of the biasing member from its initial shape and pressing the slide element toward the wheel, thereby releasing the chain from the plate and the wheel and vice versa.