The diurnal variations of odd nitrogen species (N, NO) in the thermosphere are modelled by solving the time-dependent, one-dimensional continuity equations with the diffusion transport which have been developed by OGAWA and SHIMAZAKI (1975). By comparing the calculated results with N and NO distributions which were determined in an airglow (NI 5, and mass spectrometric observations the following conclusions have emerged.(1) The quantum yield of N(2D) in the reactions energetically capable of producing the excited species should be larger than 0.5, and probably as large as 0.9. (2) The quenching of N(2D) by atomic oxygen is important at altitudes above 120km. In order to be consistent with the observation of NI 5,200A airglow, the quenching coefficient must be as large as 1X10-12cm3s-1. (3) The variability of NO density profiles obtained by various experimenters is largely interpreted to be due to the large diurnal variations of odd nitrogen expected from a large diurnal change in the thermospheric temperature which is also under the control of solar activities.