The multiferroic properties of bulk CuO are manifested in the dielectric function which can be triggered by an external magnetic field h and by the temperature T. Within a microscopic model and a Green's function technique we have calculated the dielectric function ε (k; T, h). At the magnetic phase transition temperature T_N2 the dielectric function offers a pronounced anomaly. This kink disappears when the magnetic field is enhanced and ε (k; T, h) decreases with increasing h-field. Both properties are indications for a strong magnetoelectric coupling in this material. The observation of multiferroism in CuO within an analytical approach is achieved by considering frustration and a linear magnetoelectric coupling.