In this thesis we constructed five electrochemical sensors for determination of hydrogen peroxide, methotrexate, fenitrothion, promethazine and hydrazine. In the first study, porous silicon/silver nanocomposite was prepared and used as a modifier in the carbon paste electrode, as an electrochemical sensor for the determination of hydrogen peroxide (H2O2). Silver nanoparticles were decorated on the porous silicon by using a simple and fast galvanic replacement reaction. After characterization of the nanocomposite, carbon paste electrode modified with this nanocomposite and used for determination of H2O2 in 0.1 mol L-1 phosphate buffer solution (pH 7.0). The sensor has a linear response in the rangeing of 1.65 ?mol L-1 to 0.5 mmol L-1 with a detection limit of 0.45?mol L-1 of H2O2 .The applicability of the sensor was checked using antioxidant hair color. In the second study, an electrochemical sensor based on CoFe2O4/reduced graphene oxide (rGO) and ionic liquid modified-GCE is fabricated. The results of our study showed that CoFe2O4/IL-RGO nanocomposite has synergetic effect on the oxidation of methotrexate, by decreasing the oxidation overpotential and increasing the oxidation peak current. After optimization of the experimental and instrumental conditions, using the proposed electrochemical sensor, methotrexate can be measured in the range of 0.05 to 7.5 nmol mL–1 with a limit of detection of 0.01 nmol mL–1. Relative standard deviation for five replicates measurements of 2.0 nmol mL–1 methotrexate. In the third study, GCE was modified with polyzincon. To fabricate the electrochemical sensor, GCE was immersed in 0.10 mmol L?1 zincon solutions at pH 7.0 and then successively scanned between ?1.00 to 2.20 V (vs. Ag/AgCl) at a scan rate of 70 mV s?1 for six cycles. A comparison of the electrochemical behavior of fenitrothion on the unmodified and polyzincon modified-GCE showed that in the modified electrode not only the oxidation peak current increased, but also the overpotential shifted to lower one. The DPV responses of the fenitrothion at potential about -0.60 V was used for the determination of fenitrothion. The peak current increased with increasing the concentration of fenitrothion in the range of 5 to 8600 nmol L?1 with a detection limit of 1.5 nmol L?1. Finally, the electrochemical sensor was used for the analysis of fenitrothion in water and fruit samples. In the fourth study, graphitic carbon nitride (g-C3N4) was synthesized using thermal condensation of urea at 550 °C and characterized with some techniques, suspension of multiwall carbon nanotubes (MWCNTs) and g-C3N4 was used for modification of GCE to determination of promethazine. Results show that the oxidation peak current of promethazine is significantly increased at the modified electrode. Under optimum conditions, a calibration curve of promethazine in the concentration of 0.01 to 10 µmolL-1 with a detection limit of 0.005 µmolL-1was obtained using DPV in 0.1 mol L-1 phosphate buffer solution at pH 4.0. Proposed method was used for determination of promethazine in urine and plasma samples. In the fifth study, hydrazine was determined with Cu/g-C3N4/GCE. After the synthesis of g-C3N4 it was doped with Cu. Results showed that Cu was located as ion in the g-C3N4 structure. In the presence of hydrazine, a number of copper ions are reduced by hydrazine. With sweeping the potential toward the positive potentials, these atoms were oxidized and peak current is proportional to the hydrazine concentration. After the optimizing of the experimental conditions, calibration curve of in the hydrazine concentration ranging of 0.25 to 100 µmolL-1 with a detection limit of 0.1 µmolL-1was obtained with DPV technique in the phosphate solution (pH= 8.0). The applicability of the proposed method was examined in the water samples.