In this paper, we investigated the optical characteristic of the light guide plate (LGP) with microstructures engraved by a CO2 laser. The LGP is for edge-lit backlighting of liquid crystal display. Traditionally, the microstructures on largescaled LGP are formed by screen-printing, which are hardly formed by inject molding. However, the screen-printed LGP highly scatters light and has lower optical efficiency. Therefore, a method was proposed to use the laser to directly engrave the flat surface of a bare LGP with microstructures. The engraved LGP has better optical performance due to the polished surface of the engraved-microstructures, and can be used for large-scaled application such LCD TV backlighting. We used different processing parameters to engrave the microstructures of 11 different shapes on a bare LGP and measured the cross-sectional dimensions of the microstructure. Among them, five kinds of shapes were chosen to be engraved on the bare LGPs respectively as samples. The five LGP samples were measured for spatial and angular luminance by the BM7 and Conoscope. With difference in the profiles of the microstructures, the angular distributions of light emitting from the LGP also differ. All the experimental data of the samples were compared with one another and the on-axis luminance of the best LGP sample could potentially increase 50% more than the reference sample if a proper optical film was chosen to accompany it.