This study was carried out to evaluate the effects of different levels of fermented daphnia (Daphnia magna) as a partial protein source of rainbow trout (Oncorhynchus mykiss) alevins’ diet and its effect on growth; nutrition and feed efficiency for 8 weeks. Rainbow trout alevins with an initial body weight of 0.583 ± 0.197g (Mean ± SD) after acclimating to study condition were fed in 5 treatments each with 3 replicates based on completely randomized design. Four isonitrogenous experimental diets (52.70% crude protein) including two diets contained fermented daphnia powder and two others contained raw daphnia powder with replacement levels of 20 and 30% were obtained. A diet without daphnia powder was used as control. Five Bacillus strains including: B. subtilis, B. licheniformis, B. polymyxa, B. circulans and B. laterosporus were used for fermenting of daphnia powder. The results showed the significant effect of fermented daphnia powder on growth and feed efficiency in comparison with diet contained raw daphnia powder with the same levels (P<0.05). Also the treatments fed with the same levels of fermented daphnia had the lowest feed conversion ratio than the other treatments and the control (P<0.05). The use of Bacillus strains which are able to secret extracellular enzymes and spread with producing spores in addition of fermenting of daphnia powder would be able to inoculate into the alevin gut. This issue while it’s improving nutrients of daphnia powder would increase micro flora of alevins and finally increase the growth.