A kind of effective cyclic Fenton process was investigated, in which Fe species were reused as homogeneous catalysts after the acid dissolution of Fe-containing sludge. It was observed that the degradation of phenol was abruptly accelerated (i.e., triggering effect) after an induction phase of ca. 30 min for non-first runs. The abrupt acceleration was attributed to the rapid reduction of Fe(III) by poly-phenol intermediates, such as 1,2hydroquinone, 1,4-hydroquinone and 1,2,4-trihydroxybenzene. Specially, it was found that sludge carbon (SC) could not only improve the precipitation performance of Fe and Al species, decreasing residual concentration of Fe and Al species to 0.11 mg L-1 and 4.6 mg L-1, respectively, but also reduce considerably induction phase of the degradation, promoting the advent of the triggering effect in advance. The promotion of SC was stemmed from its unique co-catalytic property, by which Fe(III) was continuously reduced to active Fe(II) and H2O2 was turned to stronger oxidizing center dot OH and O-1(2).