Structural beam of a wind turbine blade comprising a body-root and a body-trunk in the form of a box with a section decreasing towards the blade tip, comprising various piles each formed by various layers of carbon fiber impregnated with a synthetic resin, located on the upper and lower areas, intercalated between various layers of fiber glass impregnated with synthetic resin arranged along its perimeter, including between two piles, at least one layer of reinforcing material on each of the side areas enveloped by an adhesive resin film.The invention also comprises a procedure for the manufacture of the structural beam which, amongst other stages, includes the application of the layers mentioned onto a mold and the beam curing process.