Abstract: Periplaneta americana is an insect protein source, rich in protein, fat and bioactive components, which may become a potential alternative source of fishmeal for development and application. In order to study the effects of dietary fishmeal replacement by P. americana meal on the growth, biochemical indexes, disease resistance and metabolomics of juvenile Oncorhynchus mykiss, we formulated two isonitrogen and equal energy diets to replace 0% and 50% of the dietary fishmeal with P. americana meal, and had fed the juveniles with an initial body mass of 3 g for 18 weeks. At the end of feeding trial, we challenged the juveniles with Aeromonas hydrophila. The results showed that: 1) The growth performance and survival rate after the challenge in the experimental group were significantly higher than those in the control group (P<0.05). 2) The levels of serum immunoglobulin (IgM), total antioxidant capacity (T-AOC), lysozyme (LZM) in liver and catalase (CAT) in head kidney in the experimental group were significantly higher than those in the control group (P<0.05). 3) The results of metabolomics analysis showed that the differential metabolites in serum mainly involved in eight biochemical metabolic pathways, including glycerophospholipid metabolism, sphinolipid metabolism, valine, leucine and isoleucine of biosynthesis and so on. The contents of lysophosphatidylcholine (LysoPC) and phosphatidylcholine (PC) which involved in glycerophospholipid metabolism were significantly up-regulated in the experimental group. In conclusion, 50% dietary fishmeal replacement with P. americana meal could improve the growth performance, antioxidant capacity, immunity and resistance to A. hydrophila of juvenile O. mykiss significantly.