A light guide plate has a bottom surface including flat surfaces (FSs) and a plurality of prism patterns (PPs) disposed alternately with the FSs, a light emitting surface (LES) and a light incident surface (LIS). Each FS is at a first distance respectively from the LES and the first distances gradually decrease along a direction away from the LIS. Each PP has a first slanted surface (FSS) and at least a groove having a second slanted surface (SSS) and a third slanted surface (TSS). The two opposite sides of the FSS are spaced out a second distance apart. The two opposite sides of the SSS are spaced out a third distance apart. The specific value of dividing a first orthogonal projection of the second distance on an axis perpendicular to the FSs by a second orthogonal projection of the third distance on the axis is between 0.5 and 1.5.