A bipolar high-voltage pulse with a rising time of 20 ns was employed to generate diffuse dielectric-barrier-discharge plasmas in N2 and air using a wire-plate electrode configuration at atmospheric pressure. Both the discharge images and the optical emission spectra of the discharges were recorded successfully. Rotational, vibrational and excitation temperatures of the diffuse discharges in different discharge parameters and different gaseous admixtures were diagnosed using the means of optical emission spectroscopy. Rotational temperature from 300 to 510 K and vibrational and excitation temperatures from 1600 to 2400 K were measured over all conditions. It is also found that rotational temperature increases with rising pulse peak voltage, pulse repetition rate, and the concentration of admixture of Ar but decreases with rising the concentration of admixture of O2. However, vibrational and excitation temperatures vary completely opposite to rotational temperature.