Optoelectronic properties of CVD graphene are characterized over a wide frequency range: THz, IR, visible, and near‐UV. We used Raman spectroscopy to characterize the synthesized graphene films. All graphene layers were deposited on various substrates, some ones transparent or flexible, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), quartz, and silicon. Transmission Terahertz time‐domain spectroscopy (THz‐TDS) method, in the range from 100 GHz to 3 THz, is used to analyze the transmittance, sheet conductivity, and attenuation of graphene and the complex refractive index of substrates. From IR, near‐UV and visible spectroscopy we obtained the transmittance of the substrate and the sample at those frequency ranges, and we deduced the graphene transmittance on each substrate. We found that it is close to 97% in most cases.