Subject of investigation - thermal extermination of monocotyledonous weed plant (barnyard grass (echinochloa crus-galli (L.))) using hot water vapour. Ecological weed control in crops is very important for development of ecological agriculture economy. While carrying out thermal weed extermination, it was noticed that monocotyledonous weeds are difficult to exterminate (to affect) thermally. The analysis of thermal extermination of the monocotyledonous weed plant (barnyard grass (echinochloa crus-galli (L.))) using wet water vapour is presented in the work. On the basis of temperature spread regularities in barnyard grass tissues established, the main technological indexes of thermal weed extermination using wet water vapour, application of which enables the efficient and ecological control of this difficult to exterminate weed plant, were determined. The data obtained were analysed statistically using the two-factor dispersion analysis. Growth stage efficiency of barnyard grass not dependent on the thermal effect duration, thermal effect duration influence not dependent on growth stages, as well as their interaction was discussed. The aim of this work is investigation of the water vapour thermal impact on the weed plant barnyard grass (echinochloa crus-galli (L.)) in various growth stages, with the purpose of its thermal extermination.