A laser scanning unit mainly includes a semiconductor laser, a collimator, a micro electronic mechanic system (MEMS) oscillatory mirror, and an fθ lens or an f sin θ lens. The laser scanning unit MEMS oscillatory mirror is disposed between the collimator and the fθ lens to replace a conventional laser scanning unit rotary polygonal mirror for controlling a direction in which laser beams are projected from the oscillatory mirror to the fθ lens. With the laser scanning unit MEMS oscillatory mirror, the cylindrical lens may be omitted from the laser scanning unit and noises produced by the polygonal mirror rotating at high speed may be avoided. Moreover, the MEMS oscillatory mirror allows bi-directional laser scanning unit to therefore enable increased scanning frequency, simplified laser scanning unit structure, and improved laser scanning unit efficiency.