In this work, the effect of alkali and alkaline earth cations on modified silver nanoparticles AgNPs was investigated. The nanoparticles were used as a simple and low-cost colorimetric sensor for detecting alkaline earth metal cation. The determination process was based on the surface plasmon resonance properties of AgNPs. Interaction of AgNPs with alkaline earth metal cations in the presence of cysteine molecules induced a red shift in the surface plasmon resonance SPR maximum of AgNPs, as a result of nanoparticles aggregation. Consequently, yellow color of AgNPs solution was changed to pink. The examined cations were consisted of Mg2 , Ca2 and Ba2 . The calibration curve was obtained for each cation. Limit of detection, linear range and sensitivity of each cation was obtained and compared. The best detection limit and sensitivity were observed for Ba2 ions. Thus, this method was applied to the determination of barium ions in aqueous samples. In order to eliminate the interferences due to the presence of other cations, all measurements were carried out at pH 7 and in the presence of appropriate amounts of EDTA as masking agent.