In this study, the effect of antioxidant property of green macroalga Caulerpa sp. hot water extract on the prevention of rainbow trout (Oncorhynchus mykiss) viscera oil oxidation at 30°C in a 42-day period, was investigated. The treatments were conducted as a control group (fish oil without preservative), fish oil containing 100 ppm of BHT, and fish oil containing 1000 ppm of aqueous extract of Caulerpa. The results of peroxide value, p-Anisidine value, TOTOX value, and UV270 indicated that the oil containing Caulerpa extract delayed the oxidation compared to the control group, although it showed a relatively lower efficacy compared to the BHT. The Caulerpa treatment showed the highest values of peroxide and TOTOX value on the 35th day which were 38.66 meqo2/kg and 88.24, respectively. While, the BHT treatment showed 30.3 meqo2/kg and 71.70 for peroxide and TOTOX value at the end of the experiment, respectively. Moreover, p-Anisidine showed reached 13.38 and 11.04 mg/kg oil in Caulerpa and BHT treatments at the end of the experiment, respectively. The highest absorbance in 270 nm was 0.9, 0.7, and 0.6 in control, Caulerpa, and BHT treatments, respectively. The results of L* a* b* have not shown a significant difference during the storage period. In general, the results of this study showed that Caulerpa hot water extract as a natural antioxidant delayed the primary spoilage (peroxide value) for 1 week and prevented the second spoilage (p-Anisidine index) of fish oil for up to two weeks and can be offered in the feed and aquaculture industry.