In this study, antioxidant activities of acetone/dichloromethane and methanol extracts of Arctic seaweeds were investigated. The antioxidant properties of both extracts of arctic seaweed were evaluated using two different peroxynitrite tests, including scavenging power on authentic peroxynitrite and inhibitory activities on peroxynitrite generation from 3-morpholinosydnonimine (SIN-1) producing superoxide anion and nitric oxide simultaneously in vitro. At concentration of $10;{mu}g/ml$, the acetone/dichloromethane and methanol extracts of Odonthalia dentata exhibited 54.6 and 64.2% inhibition against peroxynitrite generation from SIN-1 while they exhibited 24.3 and 23.0% scavenging activities on authentic peroxynitrite, respectively. On the other hand, the acetone/dichloromethane extract of Polysiphonia stricta exhibited 61.8% inhibition only against the induced peroxynitrite from SIN-1. Furthermore, the crude extracts of Odonthalia dentata and Polysiphonia stricta were fractionated into n-hexane, 85% aq. MeOH, n-BuOH, and $H_2O$ fractions, successively, and only 85% aq. MeOH fraction exhibited the best inhibition.