The degradation of atrazine, Cationic Red X-GRL in the presence of monophenolic compound (phenol, 4-CP) by Fenton system at a low Fe2+ concentration of 7 mg·L1 was investigated, compared with that without monophenolic compound. In the absence of monophenolic compound, the degradation of atrazine was very quick at the first 2 min but then almost stopped. However, the degradation of atrazine with monophenolic compound was greatly enhanced. All atrazine was degraded within 10 min, also 4-CP degradation had a good efficiency. The enhancement of atrazine degradation was due to the higher concentration of Fe2+ in the presence of monophenolic compound. The intermediates of monophenolic compound such as hydroquinone and catechol reduced ferric ion quickly and promoted the Fenton reaction. The joint degradation of Cationic Red X-GRL with monophenolic compound also showed a similar result. This result suggested that the joint treatment of the pesticide or dyestuff-contaminated water with monophenolic wastewater to reduce Fe 2+ input was feasible.