The steady state translational energy distributions of O(1D) in the stratosphere between 20 and 50 km have been studied by laboratory experiments and Monte Carlo simulations. The results indicate that at all altitudes studied the translational energies of stratospheric O(1D) are distributed at high energies more than the Maxwell--Boltzmann distributions of the local temperatures. The predominant source of stratospheric O(1D) atoms is the solar UV photolysis of ozone, and the nascent O(1D) atoms produced have large translational energies due to the excess energy. The average translational energy of O(1D), for instance, at the altitude of 50 km, is found to be about twice as large as the thermal energy at the temperature of the ambient air. The nonthermal steady state translational energy distributions of O(1D) atoms result from the relatively slow translational energy relaxation process compared with the electronic quenching process (1D → 3P) by collisions with ambient air molecules (N2 and O2). It is suggested that the atmospheric reactions involving O(1D) atoms in the stratosphere proceed under nonlocal thermodynamic equilibrium conditions.