Further development in plasma electrolytic treatment (PET) implies the use of various procedure structures of heating which would enable to treat a larger variety of workpieces. For example, jet heating allows to modify the surface of large machine parts, as well as parts of intricate shapes since the process does not occur all over the surface but locally, i.e. at the contact point of the electrolyte jet with a workpiece where a vapour-gaseous envelope appears. It is impossible to determine energy balance in the three-phase system consisting of an anode, vapour gas envelope (VGE) and electrolyte using the method described in [1] since the assumptions made in [1] are not met. The aim of this work is to develop a method of determination of heat flux into a workpiece treated with jet plasma electrolytic heating.  The workpiece is modeled as a thin horizontally lying metal plate, whose lower surface is being heated with a constant heat flux from a round source with a preset radius, the rest of the lower part of the surface being thermally insulated. Heat exchange with the environment occurs on the upper surface according to Newton‘s law. Given the small thickness of the workpiece and its considerable length, as compared with the heating area, heat exchange on the sides can be neglected. Cylindrical coordinates system is introduced for convenience. Let us put the origin of coordinates at the center of the heating area. Therefore, two approaches can be used to determine heat distribution inside the plate. In the first case, solving Laplace problem with boundary condition, on the supposition that temperature field is axially symmetrical, we shall obtain heat distribution as a function of radial and vertical coordinates. With the second approach, small thickness and low heat conductivity allow to neglect the vertical heat flux, thus only radial temperature dependence is taken into consideration. In this case, we have to consider two areas: the round heating area and the ring area. An appropriate one-dimensional heat transfer equation should be solved for each area, boundary condition of the fourth kind being used at the area boundary. Thus we shall obtain heat distribution as a function of radial coordinates.  In order to determine the heat flux into a workpiece treated with jet plasma-electrolytic heating, we need to find out the temperature on the surface of the plate at a number of locations at various distances from the heating area. Heat flux density into the workpiece and convective coefficient of heat transfer from the plate to the environment can be calculated by means of approximating the obtained experimental data with the temperature relationship derived either from the first, or from the second approach. Using steel plates less than 10 mm in thickness provides similar experimental data with both approaches, thus the second approach appears more preferable, given the smaller amount of calculation.