$Rumex$ $dentatus$ L. (Family: Polygonaceae) is a weedy plant widely distributed in many countries including Egypt. It has been used in the Mediterranean diet as a leafy vegetable and its leaves and roots exhibited various biological activities. In our study, total phenolics, antioxidant capacities assayed by 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging and ${eta}$-carotene bleaching methods and reducing power were evaluated in different extracts/fractions of leaves and roots of $R.$ $dentatus$ grown in Egypt. In addition, their phenolic compositions were determined by GC-MS and HPLC. The results showed that total phenolic content in the ethyl acetate fractions of leaves and roots were high and measured at 169.5 and 257.4 mg gallic acid equivalent per g extract, respectively. The ethyl acetate fractions of leaves and roots exhibited strong DPPH activity and the DPPH $IC_{50}$ values were 0.021 and $0.012mg;mL^{-1}$ of leaves and roots, respectively. Furthermore, the ethyl acetate fractions of leaves and roots showed high reducing power and antioxidant activity assayed by ${eta}$-carotene bleaching method. GC-MS and HPLC analyses indicated that these fractions contained a variety of phenolic compounds including $p$-hydroxybenzoic acid, syringic acid, vanillin, benzoic acid, ferulic acid, and cinnamic acid. Our study verified that the ethyl acetate fractions of leaves and roots of $R.$ $dentatus$ have strong antioxidant activities which are correlated with its high levels of phenolic compounds and therefore, they could be utilized as a natural source of antioxidant in food industry.