Rainbow trout farms released always a large amount of organic matter into the environment, resulting in environmental contamination. If these waste can be used as a food source for the production of a secondary organism such as Nereis diversicolor, in addition to increasing production, it will reduce the volume of contamination originating from fish farms. In this regard, determining the nutritional value of the faeces produced and its effect on the growth performance of the worms feeding them compared with a standard commercial feed provides the possibility of better management to achieve maximum productivity and sustainable aquaculture. In this research, two treatments in three replicates with the density of 1000 worm/m2 were examined during 60 days under the same conditions (temperature 17.71 ± 0.6°C, oxygen 7.77 ± 0.16 mg / L, pH 7.96 ± 0.09). The first treatment was fed with faeces produced by rainbow trout and the second treatment was fed with a commercial feed of trout (Faradaneh). The results showed that worms fed with fish food had better conditions in terms of growth performance during 60 days in all growth and survival factors than worms fed with fish faeces. The main fatty acids in both groups of worms were Palmitic acid (C16: 0), Oleic acid (C18: 1 n9c), and Linoleic acid (C18: 2 n6c). Also, the ratio of DHA / EPA in both groups of worms was less than one (the first treatment was 0.40 ± 0.22% and the second treatment was 0.59 ± 0.18%). In general, the amount of main fatty acids, DHA, EPA and DHA / EPA ratio in worm samples fed with fish food were higher than those fed with fish faeces. Therefore, there is the possibility of breeding polychaete Nereis diversicolor by rainbow trout faeces, but they need more time for breeding to achieve proper growth. It is important because there is no cost for feeding.