A telescopic sight for a firearm includes a scope tube, first and second optical lenses, an eyepiece set and a linear power adjustment mechanism. The optical lenses are contained in the scope tube and configured to move along a longitudinal axis of the scope tube. The eyepiece set is coupled to one end of the scope tube. The linear power adjustment mechanism adjusts zooming power of the telescopic sight, and includes a zoom cam tube and a lever that are contained in the zoom cam tube. The lever is coupled to rotate the zoom cam tube. When actuated, the lever moves linearly along a direction parallel to the longitudinal axis of the scope tube such that a distance between the first and the second optical lenses along the longitudinal axis of the scope tube to change to adjust a zooming power of the telescopic sight.