a chain saw includes a housing , a sprocket wheel which is provided in the housing for driving a saw chain , and a chain guide for guiding the movement of the saw chain . the chain guide extends out longitudinally from the housing and is movable longitudinally relative to the housing . the chain saw includes a chain tensioning device . the chain tensioning device comprises a driven member arranged on the chain guide and a driving member which drives the driven member to move the chain guide longitudinally and a rotating device rotating the driving member . a stepless clutch device is arranged between the rotating device and the driving member . the chain tensioning device presses the chain guide while it tensioning the saw chain . thus it prevents effectively the chain guide from loosing after the saw chain is tensioned . the operation of the chain tensioning device is labour - saving . the chain tensioning device has a stable structure and it is hard to be worn .