The quantitative correlations between the chemical structure and the antibacterial activity against Staphylococcus aureus 209P, Bacillus subtilis PCI-219, Escherichia coli NIHJ and Salmonella typhimurium of 19 N-substituted N-thioformylhydroxylamine derivatives have been studied by Hansch-Fujita's method. These compounds are analogues of an antibiotic thioformin, N-methyl N-thioformylhydroxylamine. The following parameters were used in this study ; Hansch's π and chromatographic Rm values as hydrophobic parameters and pK_a value as an electronic parameter. As the result, in the N-substituted N-thioformylhydroxylamine series, the hydrophobic character of the molecule as expressed by π or Rm is closely correlated to the antibacterial activity of these compounds.