Tilapia is a fishery commodity and plays a role in supporting national food security. The development of tilapia aquaculture needs to be carried out, including the development of feed. Feed fortification with other ingredients can reduce the production costs of tilapia aquaculture. Fortification that can be done includes using seaweed. Seaweed is an algae that is used as raw material in various industries because seaweed has good nutritional content. This study aims to analyze the effect of fortification of fish feed with Gracilaria sp. on growth, survival, feed efficiency and FCR of cultured tilapia. The research method used is an experimental method. This study used a completely randomized design (CRD) using four (4) treatments of fortification of fish feed with Gracilaria sp. Seaweed flour. in the cultivation of tilapia (Oreochromis niloticus) with 3 replications, namely the P1 Control treatment (without the addition of Gracilaria sp. seaweed flour) P2 (Gracilaria sp. 4% seaweed flour), P3 (Gracilaria sp. 8% seaweed flour) , P4 (Gracilaria sp. 12% seaweed flour). Data were analyzed using analysis of variance (ANOVA) at a significant level of 5% with a confidence interval of 95% and continued with the Duncan test. The results showed that the absolute length growth of tilapia during 42 days of rearing with artificial feeding at various fortification concentrations of Gracilaria sp. Seaweed flour. ranged from 8.43 cm -1.2cm. Fortification treatment of Gracilaria sp. 8% (P3) gave the highest average value of growth in absolute weight, namely 32.17 g. The specific weight growth rate of tilapia ranged from 1.34% / day -1.74% / day. Tilapia FCR ranged from 1.79% -2.31% and the efficiency of feed utilization ranged from 43.37% -56.29%. The survival rate for tilapia ranged from 73.3% -76.7%. The conclusion of this study is the fortification of fish feed with seaweed flour Gracilaria sp. can affect the absolute growth of tilapia (Oreochromis niloticus), but does not affect the specific growth rate, FCR value, feed efficiency and survival rate.