Betaine and L-carnitine are important supplements in aquaculture industry. The present study was conductedto evaluate the interaction effects of these supplements and oil type on the growth performance, digestive enzymes activity and stress resistance of rainbow trout fingerlings. Four hundred and twenty healthy rainbow trout (initial weight: 9.9±0.1 g) were used. The sample was fed with four treatments (fish oil, fish oil + Betaine and L-carnitine, corn oil, corn oil + Betaine and L carnitine) and three replicates. The amount of Betaine and L-carnitine used in the present study were 500 and 1000 mgr, respectively. Results of two- way ANOVA showed that experimental variables (the supplements and oils) did not affect fish weight gain independently while their interaction effects were significant (p<0.05). Also, the results did not show a significant independent effect of the supplements on the digestive enzymes activity (p>0.05), but significant interaction effect of the supplements and oil was observed on amylase and alkaline phosphatase activity. In addition, the interaction and independent effect of variables on the resistance to environmental stress (salinity, oxygen and thermal) was investigated in fingerlings and the results revealed that oil variable did not affect stress resistance. However, the food supplements, both independently and in interaction with oil, increased fish resistance against thermal stresssignificantly.