The paper presents the results of an experiment on the effect of cold plasma (He + O-2 and He + Air) generated in a radio frequency cold atmospheric plasma jet on the process of germination of onion cv. Wolska (Allium cepa L.) seeds. In order to determine the impact of the gas flow on the seed surface, CFD simulations and Schlieren imaging were performed and surface characteristics were studied using a digital microscope. Plasma treatment of the seeds was carried out four times (2, 5, 10 and 15 s) with one control group. Pre-sowing plasma stimulation of seeds improved the germination capacity and germination energy for all tested groups in comparison to the control. The best germination capacity and energy were obtained for seeds stimulated with 10 s of plasma treatment. Analysis of the data showed a statistically significant impact of plasma treatment on the onion seed's germination parameters. Plasma treatment did not induce significant changes on the seed surface, as microscope images showed but length of all plasma-treated seedlings increased in comparison to control.