In this work, decomposition mechanism of the phenylaminyl radical C6H5N center dot H was investigated by means of M062X, G4 and CBS-QB3 methods. It is found that the energy barrier for conversion of most stable N-centered radical to less stable C-centered radicals is greater than the decomposition reaction. Two resonantly stabilized radicals propargyl and cyclopentadienyl were found on the potential energy surface of reaction. It is estimated that activation energy for sigmatropic H transfer rearrangement of cyclopentadienyl radical is 59.50 and 56.26 kcal/mol at G4 and M06-2X/6-311++ G(d, p) levels, respectively. The Delta H degrees(f, g) and Delta G degrees(f, g) values of all the radicals were calculated.