A single-crystal ESR study of the magnetic properties of N-[(2,4-dichlorophenyl)thio]-2,4,6-triphenylphenylaminyl radical crystals is reported. The stable radical crystal shows a one-dimensional antiferromagnetic behavior which has described in terms of an alternating linear chain model with J/k(B) = -7.5 K and an alternating parameter a = 0.55. A detailed analysis of the angular and temperature dependence of the ESR linewidth and g-value has been carried out. The angular dependence of the line-width and line shape observed at 5 K has shown characteristic features expected for a one-dimensional magnetic system : (3cos(2) theta-1)(4/3) angular dependence of line width and Lorentzian line shape at theta = 55 degrees. Nagata’s theory has applied for the analysis of the temperature dependence of the g-shift. However, the temperature dependence has been only qualitatively reproduced by the theory which is proposed for the one-dimensional regular Heisenberg model. We discuss on the origin of this unsatisfactory agreement. The magneto-structural correlation is also discussed based on the X-ray crystallographic and single-crystal ESR data.