An in-line laser scanning unit (LSU) with multiple light beams is disclosed. The LSU includes a minimized in-line mirror set composed by a plurality of vertically stacked Micro Electronic Mechanical System (MEMS) oscillatory mirrors and a linear corresponding scanning lens set formed by a plurality of F-Sin θ lens stacked vertically so as to correct the variation of reflective angle of the oscillatory mirror that is sinusoidal in time. Thus the scanning speed of multiple laser beams on the image plane is constant. Therefore, the volume of color printers is effectively reduced and the scanning efficiency is improved when the LSU in accordance with the present invention is applied to the optical engines of color printers.