A prism sheet, capable of enhancing wet-out ability in a whole back light unit while maintaining center brightness, transmittance and haze in good condition, is disclosed. The prism sheet includes a base film made of a transparent material and having a flat first surface, and a light refracting part including a plurality of unit prisms unitarily formed with the base film on a second surface of the base film, opposite to the flat first surface. The unit prisms have a predetermined shape of a cross section, and are arranged with orientation. When an angle formed between a ridge of the unit prism of the light refracting part and a normal line of the flat first surface is defined as θn, θn and θn+1 have different values from each other, and n is an integer more than 2. An LCD back light unit using the above prism sheet is also disclosed.