A sensitive and selective method for determination of mercury(II) with "4-(4-methylphenyl aminoisonitrosoacetyl)biphenyl (TKO)-modified pencil graphite electrode" was developed. All factors affecting determination process were optimized. Differential pulse voltammetry with 4-(4-methylphenyl aminoisonitrosoacetyl)biphenyl-modified electrode showed a linear response between 1.0 x 10(-5) and 1.0 x 10(-3) M (R (2) = 0.9994). The detection limit of this electrode was found as 5.85 x 10(-7) M (S/N = 3). The effects of different cations on the determination of mercury(II) were investigated and found that modified electrode is highly selective. The developed method was applied for mercury determination in different water samples.