Abstract: Metham-sodium is a methylamine derivative which is used on cucumber, tomato, and tobacco (seed bed). It can protect crops from soil-borne diseases, but it will affect soil microbial, then affect soil enzymes. This study was an attempt to evaluate the environmental ecology influence after using metham-sodium. By simulation in laboratory, the investigation studied the relationship between different concentrations of metham-sodium, different treatment time and soil microbial population, soil enzyme activities in a selected soil. The results showed as follows: metham-sodium had similar effects on soil bacteria and actinomycetes which were inhibition-stimulation-recovery, but the effect on bacteria was bigger than actinomycetes. The highest stimulation of bacteria was 293.61%, however, the actinomyces was 180.77%. While metham-sodium at each concentration inhibit soil fungi at the test; the inhibition of high concentration (90.28 mg/kg, 180.56 mg/kg) was 100%, and the level of inhibition was proper to the concentration of metham-sodium. The effects of four representative soil enzymes to metham-sodium showed different dynamic changes. The effects of metham-sodium on soil invertase were dosage-dependent. When soil was fumigated with low or medium concentrations of metham-sodium, the effects showed stimulation-inhibition-stimulation-recovery, while those treated with high concentrations of metham-sodium showed inhibition-stimulation-recovery, and the stimulation was up to 213.62%. To all of the concentrations of metham-sodium, soil urease activity presented irreversible inhibition impact, and the inhibition of metham-sodium on soil fungal were dosage-depenndent. The effects of metham-sodium on soil phosphatase and catalase were inhibited at first, and then stimulated. It showed that metham-sodium have great effect on soil fungi and urease, but to soil bacteria, actinomycetes, soil invertase, phosphatase and catalase, the effects are temporary without potential harm to soil fertility and crop growing.