Reported here is the study on various types of ESR spectra of the propagating radical in polystyrene. Spectrum simulation, analysis of the hyperfine coupling constant, and calculations of the conformation energy show that the propagating radical of styrene has two stable inner-rotation conformations R1 and R2. R1 corresponds to a B-line spectrum which, in some special conditions, can appear as a 4-line or 3-line spectrum. R2 corresponds to an 8-line spectrum which, in some special conditions, appears as a 6-line spectrum. All the ESR spectra observed in the polymerization process can be regarded as the superposition of some of these spectra. The concentration ratio of R1 to R2 is affected by rotational interchange between R1 and R2, as well as the propagating reaction. The rotational interchange between R1 and R2 favors the formation of R1, and the propagating reaction favors the formation of R2. At the beginning stage of polymerization (low monomer inversion stage), the rotational interchange between R1 and R2 is dominated and the value of the concentration ratio of R1 to R2 is consistent with the one given by the Boltzmann distribution. However, in the middle to end stage of the reaction (high monomer inversion stage), the propagating reaction dominates. Therefore, R2 is the major conformer at last. Furthermore, when the radical is in the solid matrix in which the monomer has almost been exhausted and the propagating reaction is close to stopping, the rotational interchange between R1 and R2 dominates again. Therefore R1 returns as the major conformer in the solid matrix.