Abstract The aim of this paper is to describe real course of the depth of heat affected zone in sub-surface layers of the samples made by technology of wire electrical discharge machining with brass electrode. It is vital to remark in the beginning that the heat impact is an undesirable phenomenon from the standpoint of operating life of parts produced by this progressive technology. Considerable depth of heat affected zone, its characteristic, and its uneven course along cross-section, noticeably contribute to the decrease of quality and longevity of machine parts, as well as cutting tools. The samples were produced on electroerosion machine AGIECUT from tool steel of 950 MPa strength. Applied cutting tool was wire brass electrode of 0.25 mm diameter. To determine suitable HAZ depth measuring method it was necessary to take into account properties of the given technology where impact and consecutive micro-hardness change occurs instantly after high temperature spot effect of electric discharge channel which arises between wire electrode (cutting tool) and material (workpiece) during intensive cooling in dielectric medium. Considering this aspect, the influence shows different values in longitudinal and lateral cross-sections (as of wire electrode movement relatively to the workpiece). The course of the values is in the same time greatly affected by combination of technological parameters that inevitably must observe electroerosion cutting process itself, as well as electro-chemical properties of the material.