The invention relates to a plasma source operating at atmospheric pressure, to be used for treatment of materials. According to the invention, the plasma source is assembled of two external electrodes (7) and an internal electrode (5), confining two narrow flat discharge spaces (4) which are supplied with the gas in which the plasma is generated and open towards the lower part of the source with two rectangular nozzles wherethrough the plasma expands as planar jets (14), the internal electrode (5) playing the role of hot electrode connected to the power terminal of a radio-frequence generator, having mounted therein a precursor gas injection chamber (6), where the surfaces of the external electrodes (7) are separated by some barriers (8) of dielectric material, the said spaces being laterally closed with some electrically insulating spacer elements (9, 10) which prevent gas from leaking out of the source.