The invention relates to a compound semiconductor wafer, a light emitting diode, and a preparing method thereof. The light emitting diode includes a compound semiconductor crystal layer (2) includingan emission layer (22) and a conductive substrate (6) bonded to the crystal layer (2) through a metallic layer (5). The metallic layer (2) includes a first metallic layer (51) formed on one principalsurface of the compound semiconductor crystal layer (2), a second metallic layer (53) formed on one principal surface of the conductive substrate (6), and a metallic microparticle layer (52) composedof metallic microparticles which are 1 nm to 100 nm in average diameter and bonded to each other between the first metallic layer (51) and the second metallic layer (53).