Thermal stabilization of polyacrylonitrile (PAN) multifilaments was performed using a multi-step annealing approach in an air environment. Ammonium persulfate (APS) pretreatment of PAN multifilaments was performed prior to the thermal stabilization process. APS pretreatment of PAN resulted in the sulfation of polymer chains. The results suggest that APS pretreatment accelerated the thermal stabilization reactions of the PAN multifilaments. Thermal treatment of APS-treated PAN multifilaments resulted in enhanced thermal stability. It seems that APS promoted the oxidative cross-linking of polymer chains. The structure and properties of thermally stabilized samples were determined employing a set of measurements, such as fiber thickness, linear density, volume density, flame test, mechanical properties, X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared (IR) spectroscopy, and scanning electron microscopy (SEM). Analysis of the X-ray diffraction and FT-IR spectroscopy results suggested rapid aromatization reactions with increasing stabilization times. Detailed analysis of X-ray diffraction curves obtained after a curve-fitting procedure suggested the rapid transformation of the original structure to a totally disordered amorphous phase containing pre-graphitic domains as shown by the gradual reduction in the degree of apparent crystallinity of the original PAN multifilament. The experimental results suggest that ammonium persulfate was very effective and efficient in promoting stabilization reactions. The use of APS is expected to reduce the overall manufacturing cost of the final carbon fiber processing by reducing the time needed for the thermal stabilization step.