An electrochemical assay for sensitive and selective determination of mercury was suggested. Preparation, characterization, and testing the electro-catalytic activity of Crown-ether/multi-walled carbon nanotubes (MWCNTs) / CPEs were performed.  The voltammetric responses of modified electrodes with various sensing elements and nanomaterials were carefully investigated. Consequently, improving the sensitivity and selectivity were achieved using a combination of 12-crwon-4- ether and MWCNTs (10% w/w for each). A linear response was observed from 5 -110 µg/ml, with the detection limit (S/N=3) of 0.25 µg/ml. The method was interference free from many species and successfully applied for the determination of mercury in blood, urine or tab-water samples with a high accuracy and precision comparable with the reference method (Atomic Absorption Spectroscopy AAS).