In this research, it has been used nanomaterials and nanostructures for the modification of working electrodes to improve efficiency and sensitivity for the electrochemical sensor. Carbon dots (C-dots) have recently attracted extensive interest for their unique properties of tunable photoluminescence, low toxicity, and favorable biocompatibility. In this work, in the first section, highly amino-functionalized fluorescent carbon-dots (CDs) were fabricated by hydrothermal carbonization of chitosan at a mild temperature. Multiwall carbon nanotubes (MWCNTs) and Carbon dots (CDs) were used in the preparation of working electrode. This sensor was based on poly (diallyl dimethyl ammonium chloride) (PDDA) / Multiwall carbon nanotubes (MWCNTs)-Carbon dots (CDs) modified pencil graphite electrode (PGE) for monitoring the dextromethorphan oxidation signals. Field emission scanning electron microscopy (FE-SEM) was employed for characterization of the various modifiers that were formed at the electrode surface. The results showed that the chemically modified electrode exhibited considerable sensitivity. The electrochemical behavior and determination of dextromethorphan have been studied on the PDDA/MWCNTs-CDs/PGE by cyclic voltammetry and differential pulse voltammetry techniques for the sensitive determination of dextromethorphan in the plasma and urine. Different parameters on the response of measurement of dextromethorphan are optimized. Under the optimized conditions using differential pulse voltammetry one linear range was obtained from 2.0 ?M – 600.0 ?M for dextromethorphan, with a detection limit of 0.19 ?M. results show that the sensor is a useful tool for determination of dextromethorphan in biological samples.