Abstract: A chain saw automatic tensioner preferably includes a base plate, a chain bar, a tensioning cylinder, a sliding block and a pusher block. The base plate includes a sliding slot that is sized to slidably receive the sliding block. The chain bar includes a substantially half oval shape that is sized to retain a cutting chain. An end of the chain bar is retained between a top of the sliding block and a bottom of the pusher block. The tensioning cylinder is mounted to the base plate opposite the sliding slot. Hydraulic oil in the tensioning cylinder forces a cylinder rod and the pusher block secured to the chain bar away from a chain sprocket and thus tensioning the cutting chain.