The effect of hot deformation process on the structure within the temperature range corresponding to the g→a+P transformation was tested for commercial carbon steel containing of 0.158% C. Hot compression tests were performed at computer controlled constant cooling rate. Before testing CCT diagrams were carefully determined in order to fit hot deformation conditions to the g→a+P phase transformation range. The material microstructure was examined by means of optical and electron transmission microscopy. It was found that ferrite, pearlite or bainite were typical microstructural components observed at hot deformed samples. Localized distribution of pearlite or bainite that resulted from flow localization was observed rarely. Precipitation of carbides on ferrite grain boundaries is the most typical microstructural effect of the hot deformation/precipitation interaction that was noted for the most of hot deformed samples.