The aims of the work were to investigate effect of NH₄⁺ on Arabidopsis seedlings of different accessions growing in agar medium or hydroponics in root characteristics, morphology and physiology.Results indicate primary root length, lateral root length and root surface area of Arabidopsis all decreased with increasing NH₄⁺, but root average diameter went up first and then down with increasing NH₄⁺.Difference between ecotypes of Arabidopsis in response to elevated NH₄⁺ was obvious, but no similar phenomenon was found when the seedlings were exposed to the same concentration of K+.Further study on two significantly different accessions of Arabidopsis (Col-0 tolerant to NH₄⁺ and JA22 sensitive to NH₄⁺) show that the effect of NH₄⁺ was significantly lower on Col-0 than on JA22 in inhibiting root length, and so was it in promoting root average diameter, but no much difference was observed between them.Treated with 30 mmol L^-1 NH₄⁺, Col-0 was significantly higher than JA22 in biomass of the shoots, but much lower in chlorophyll content, and they are more or less the same in water content.And in terms of NH₄⁺ uptake rate, Col-0 was at least 3 times higher than JA22.Therefore, it could be concluded that the Arabidopsis growth inhibited by NH₄⁺ was not attributed to rhizosphere acidification, absence of NO₃^-, high ion intensity or high availability of NH₄⁺ to plants.