The disclosure relates to a viewing angle switchable back light unit wherein a general mode and a privacy mode in a liquid crystal display can be selected. A thin film type back light unit includes a light guide film, a light radiator, a first light source, and a light collimator. The light guide film includes a light entering part at one side, a light guiding part extending from the one side to an opposite side of the one side, and a light radiating part on one plane surface. The light radiator is on the light radiating part. The first light source faces the light entering part. The light collimator is on the light entering part facing the first light source. The light collimator receives an expanding light from the first light source, converts the expanding light into collimated light, and provides the collimated light to the light entering part.