A simple and sensitive electrochemical sensor for simultaneous and quantitative detection of ranitidine (RT) and metronidazole (MT) was developed, based on a poly(thionine)-modified anodized glassy carbon electrode (PTH/GCE). The modified electrode showed the excellent electrocatalytic activity towards the reduction of both RT and MT in 0.1M phosphate buffer solution (PBS, pH 7.0). The peak-to-peak separations (${Delta}E_p$) for the simultaneous detection of RT and MT between the two reduction waves in CV and DPV were increased significantly from ca. 100 mV at anodized GCE, to ca. 550 mV at the PTH/GCE. The reduction peak currents of RT and MT were linear over the range from 35 to $500{mu}M$ in the presence of 200 and $150{mu}M$ of RT and MT, respectively. The sensor showed the sensitivity of 0.58 and $0.78{mu}A/cm^2/{mu}M$ with the detection limits (S/N = 3) of 1.5 and $0.96{mu}M$, respectively for RT and MT.