The contribution of the Unresolved Extragalactic Radio Sources to the diffuse brightness of the sky was evaluated using the source number - flux measurements available in literature .
We first optimized the fitting function of the data based on number counts distribution .
We then computed the brightness temperature at various frequencies from 151 MHz to 8440 MHz and derived its spectral dependence .
As expected the frequency dependence can be described by a power law with a spectral index \gamma \simeq - 2.7 , in agreement with the flux emitted by the steep spectrum sources .
The contribution of flat spectrum sources becomes relevant at frequencies above several GHz .
Using the data available in literature we improved our knowledge of the brightness of the unresolved extragalactic radio sources .
The results obtained have general validity and they can be used to disentangle the various contributions of the sky brightness and to evaluate the CMB temperature .