Promethazine modified carbon paste ion selective electrode has been prepared, based on the ion pair of promethazine hydrochloride Pm.Cl with sodium tetraphenyl boron(III) Pm-TPB dissolved in dioctyl phthalate DOP as a pasting liquid. The electrode showed the linear response with mean calibration graph slope of 58 mV/decade at 25℃; at a concentration range and a lower detection limit of 1 × 10^-5 -6 × 10^-2 M and 8 × 10^-6 M promethazine hydrochloride, respectively. The change of pH of the tested solution within the range 4.2 -6.8 did not affect the electrode performance. The electrode showed a very good selectivity toward Pm+ cations with respect to a large number of organic and inorganic cations and compounds. The standard addition and potentiometric titration methods were applied for the determination of promethazine hydrochloride in pure solution and in pharmaceutical preparations with an average recovery range of 97.3% -101.4% and a mean relative standard deviation (RSD) of 0.35% -1.83 % which were of comparable accuracy and precision to the previously reported works.