A highly sensitive and selective optical membrane sensor was prepared for the determination of ultra trace amount of Ni 2+ ions. The plasticized PVC membrane incorporating dibuthylphthalate and 2-amino-1-cyclopentene-dithiocarboxylic acid (ACDA) as a chromoionophore works on the basis of a cation-exchange mechanism and shows a significant absorbance signal change on exposure to acidic solution containing nickel(II) ion. The sensor displays a calibration response for Ni 2+ ion over a wide concentration range of 3.1 × 10 −8–8.0 × 10 −3 M, and a response time of 3 min. In addition to high stability, reproducibility and relatively long working lifetime, the sensor possesses good selectivity for nickel(II) ion over several common diverse ions. The sensor was successfully applied to determine the traces of Ni 2+ ion in some water samples.