We report a process using NiFe2O4 nanocatalyst for the heterogeneous activation of sulfite ions for estriol (E3) oxidation. This NiFe2O4-sulfite system consisted of NiFe2O4 as an catalyst, sulfite as a reductant, O-2 as an oxidant, and oxysulfur radicals (SO3 center dot-, SO4 center dot- and SO5 center dot-) and hydroxyl radicals (HO%) as generated radicals. The experimental conditions for E3 removal were investigated, including the initial pH values, dissolved oxygen concentration, and NiFe2O4 and sulfite ion dosages. The initial pH has a significant influence on the catalytic performance, and approximately 87.6% of E3 could be removed at pH 9.0. NiFe2O4 nanocatalyst was characterized by high resolution and field emission transmission electron microscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy. The catalytic reactivity and stability of the NiFe2O4 nanocatalyst were investigated by consecutive repeated runs. Radicals generated in this NiFe2O4-sulfite system were identified via radical-scavenging and electron spin resonance experiments. SO5 center dot- was identified as major oxidative radicals for E3 removal, which is quite different from the previous studies on sulfite activation. The generation of > SO32--Ni(II)-OH on NiFe2O4 surface played a critical role in sulfite auto-oxidation. Our results supplemented the mechanism of the sulfate radical generation with sulfite ion as a source.