Background: Providing additional food in the form of cookies is one of the efforts to reduce the problem of malnutrition in toddlers. Sago is one of Indonesia's local foods as a raw material for cookies, which is containing high carbohydrates but low in protein. It is necessary to substitute sago with mackerel, which is containing high protein and linolenic acid. This study aims to determine the effect of mackerel flour subtitution on the acceptability and energy protein content of sago cookies. Methods: This was an experimental study with a completely randomized design. There were four variations of mackerel flour substitution, namely 0% (A) as control, 5% (B), 10% (C), 15% (D). The substitution of mackerel flour as independent variable, while the dependent variables were an acceptability, energy content, protein content. Data were statistically analyzed using ANOVA. Results: Statistical test to determine the difference in sago cookies showed p-value = 0,860 for the acceptability, p-value = 0,001 for the energy content, p-value = 0,000 for the protein content. Conclusion: The substitution of mackerel flour has a significant effect on the energy and protein content, but it has no effect on the acceptability of sago cookies.