Nitrogen-doped polycrystalline diamond (N-PCD) is one of the most important carbon-based electronic materials. Here we present a systmatic investigation of the effect of high-temperature annealing (HTA) on basic physical properties of N-PCD wafer grown by microwave plasma-enhanced chemical vapor deposition (MPECVD). The metallographic and optical microscopes, X-ray diffraction,Raman spectroscopy and X-ray photoelectron spectroscopy are applied for the characterization of N-PCD before and after HTA. Particularly, we study the optoelectrnic properties of N-PCD before and after HTA by using terahertz (THz) time-domain spectroscopy. THz transmittance, dynamical and static dielectric constants and optical conductivity for N-PCD are measured. For N-PCD before HTA, we can observe a THz absorption peak induced by weak H-bonds in N-PCD, which was predicted theoretically in 1999. For N-PCD after HTA, we find that the corresponding optical conductivity can be rightly descriped by the Drude-Lorentz formula. Thus, via fitting the experimental results with the theoretical formula, we can determine optically the key electronic parameters of N-PCD after HTA, such as the electron density, the electronic relaxation time and the Lorentz frequency. The temperature dependence of these parameters is examined in the range of 80–300 K. This work demondtrates that HTA is a practical and efficient way in improving the electronic and optoelectronic properties of N-PCD. The results obtained from this research can benefit an in-depth understanding of the effect of HTA on physical properties of N-PCD from a viewpoint of semiconductor physics.