Since many macroalgae are a rich source of antioxidants, in this paper we investigated the antioxidant potential of extracts (obtained by microwave-assisted extraction) derived from Gracilaria pygmaea red macro algae collected from the Persian Gulf coast in Bushehr (south west of Iran) using voltammetric method as an alternative to spectroscopic methods. The differential pulse voltammetric (DPV) technique was used to evaluate the phenolic content, antioxidant content, DPPH radical scavenging ability, and then the results were compared with spectroscopic methods. For this purpose the gold nanoparticles synthesized by green method were used for surface modification of carbon screen printed electrode and preparation of electrochemical sensors. To investigate the ability of algae extract to inhibit oxidizing compounds, ferrocene was used as target species and IC50 were calculated for this extract against several conventional oxidants. By the use of the electrochemical sensor, the phenolic and antioxidant contents of the extract were calculated 2.24 and 0.8 mg/g respectively. The results showed that Gracilaria pygmaea algae extract has a good ability to inhibit oxidizing and radical compounds. Also, the anticancer activity of the extract was evaluated on HT-29, SKOV3, MCF-7, and SKMES-1 cell lines using MTT assay. (C) 2019 The Electrochemical Society.