Abstract: A grease gun with an observation window is provided with a metal outer barrel, a plastic transparent inner barrel received in the outer barrel, and an operating end cover and a piston end cover screwed at two ends of the outer barrel. The observation window is formed on the outer barrel, and the two end covers define an oil storage space with respect to the inner barrel, and the interior of the oil storage space can be seen through the observation window of the outer barrel and the transparent inner barrel.