Translational relaxation of suprathermal N(4S) in collisions with N2, O2, He and Ar, which is relevant to nitric oxide formation in the thermosphere, has been studied experimentally. The suprathermal N(4S) atoms which have an average translational energy of 0.93 ¿ 0.10 eV (21.4 ¿ 2.2 kcal mol-1) in the laboratory frame were produced by 193 nm photolysis of NO2. Doppler profiles of the N(4S) atoms were recorded by laser-induced fluorescence detection of N(4S) around 120.07 nm, from which the average kinetic energy of the N(4S) atoms were obtained as a function of thermalization time. Monte-Carlo calculations employing an elastic hard-sphere collision model have been performed to estimate the hard-sphere collision radii and thermalization cross sections. The thermalization cross sections, which reproduced the experimental results, were (3.2 ¿ 0.4), (2.8 ¿ 0.4), (1.8 ¿ 0.2) and (2.3 ¿ 0.2) in unit of 10-15 cm2 for N(4S) + N2, O2, He and Ar, respectively.