A hair trimmer with a trim length adjustment mechanism includes an elongated main body; a cutter block having a stationary blade and a movable blade, both of which are comb-shaped and slidably in contact with each other, the cutter block being protrudingly installed at one end portion of the main body; a comb-shaped cylindrical comb attachment for trim length adjustment; a connecting member disposed in the main body and engaged with the comb attachment; a rotary dial, capable of transmitting its turning movement to the connecting member, installed at one side of the main body approximately in parallel with said one side; and an operation switch, disposed over a portion of the main body where the rotary dial is installed, for turning on and off an operation of the main body.