A first-principles technique capable of describing the nearly excited states of semiconductors and insulators, namely the modified Becke - Johnson (mBJ) potential approximation, is used to investigate the electronic band structure and optical properties of spinel oxides: GeZn 2O 4. The predicted band gaps using the mBJ approximation are significantly more accurate than the proposed previous theoretical work using the common LDA and GGA. Band gap dependent optical parameters, like the dielectric constant, index of refraction, reflectivity and optical conductivity are calculated and analyzed. The results from the dielectric constant shows that the numerical value of the static dielectric, after dropping constantly, becomes less than zero and the material exhibits metallic behavior. The refractive index also drops below unity for photons higher than 18 eV, which indicates that the velocities of incident photons are greater than the velocity of light. However, these phenomena can be explained by the fact that a signal must be transmitted as a wave packet rather than a monochromatic wave. This comprehensive theoretical study of the optoelectronic properties predicts that these materials can effectively be used in optical devices. © 2012 Chinese Physical Society and IOP Publishing Ltd.