A novel and sensitive method for the enzymatic determination of trace amount of mercury ion based on its inhibitory action on urease is proposed. Mercury(II) inhibits the urease-catalysed urea hydrolysis by binding with sulphydryl groups of urease. The inhibited enzymatic reaction is monitored in-situ with a SAW/impedance enzyme transducer which is composed of a SAW resonator oscillating at 61 MHz and a pair of parallel electrodes. The inhibition mechanism of Hg(II) on urease is discussed. The kinetic parameters and inhibition parameters are estimated. The frequency response of the transducer is proportional to mercury concentration over the range of 50-350 ng/mL. The detection limit is 20 ng/mL. The relative standard deviation was 2.9% for six measurements of 150 ng/mL Hg(II). The effect of potential major interferences was investigated. The method was successfully applied to the determination of Hg(II) in waste water and the results obtained by proposed methods compared well with those obtained by atomic-absorption method.