An incubation test on the soil microbial respiration as affected by 0, 5, 20, and 100 mg x kg(-1) of metolachlor and S-metolachlor (Dual Gold) showed that at the early period of incubation, 5 mg x kg(-1) of metolachlor had a stronger stimulation effect on the respiration than 5 mg x kg(-1) of Dual Gold, but after then, metolachlor presented an inhibition effect while Dual Gold had the effects of stimulation or recovery. 20 mg x kg(-1) of Dual Gold had little effect on the respiration, while metolachlor with the same concentration stimulated it. 100 mg x kg(-1) of metolachlor had a stronger stimulation effect on the respiration than 100 mg x kg(-1) of Dual Gold, but both of them had the same inhibition effect during the later period. The higher the concentration of metolachlor or Dual Gold, the greater the stimulation effect was. There was a significant difference in soil microbial respiration between treated and blank soil at the beginning of incubation, while no significant difference was observed during the later period. With the same concentration of 20 or 100 mg x kg(-1), metolachlor had more harm than Dual Gold. It was suggested that metolachlor and Dual Gold belonged to lower poisonous pesticides, and had less harm on soil microbes.