In this study, Na+ -selective potentiometric PVC-membrane electrode was developed with the commercially purchased ionophore substance to be used to determine sodium concentration in water samples. The potentiometric performance characteristics of the electrode were determined and its suitability for environmental analysis was investigated. For the measurements taken with the Na+ -selective electrode, a potential difference of 50,8 (±1,4) mV/pNa was observed for each 10 fold concentration change against the main ion solution The electrode was determined to exhibit a linear behavior towards the main ion solution in the concentration range of 1x10-1 -1x10-5 mol L-1 and was highly selective towards the tested ions. Micro dead volume flow cells were prepared in the laboratory and the sodium ion determination was made directly in environmental water samples by using these electrodes developed as a detector in the flow injection analysis system. In addition, sodium ion determination in water samples was made using the assay method developed using the standard addition method and all measurements were given in comparison with potentiometric results. The results show that the Na+ -selective micro-potentiometric PVC-membrane electrode can effectively be used for routine determination of sodium ion in environmental samples.