Abstract: The technologies of anaerobic culture case,gas chromatography and most probable number method were used to investigate effects of methamidophos and acetochlor(0,0.1,0.2,0.5,and 1.0 mg·L^-1) on population and activity of methanogens in paddy soil.The results show that the population of methanogens increased by 0.2-44.1 times,and the production of methane per tube by 0.8-2.1 times within 7 days after methamidophos was added,indicating that the stimulating effect rose with the concentration of the chemical.But the growth of methanogens was inhibited within 7 days after acetochlor was added,with the population decreased by 50%-99%,and the production of methane per tube falling significantly.And the retarding effect increased with the concentration of the pesticide.And the population and activity of methanogens returned to the normal level with the days passing on after the 7th day.