The aims of this study were to (i) investigate the ability of Icelandic brown algae Fucus vesiculosus extracts to inhibit lipid oxidation in granola bars fortified with fish oil-in-water emulsion; (ii) investigate whether addition of the seaweed extracts affected the physical microstructure of the oil droplets in granola bars. The oxidative stability of the bars at 20°C was evaluated over a period of 10 wk by measuring the development of peroxides and volatile compounds using dynamic headspace gas chromatography mass spectrometry (DHS GC-MS). The physical microstructure was determined using microscopy. All extracts − except water extract in low concentration − reduced lipid oxidation during 10 wk of storage when added in a concentration of 0.5 or 1 g extract/100 g emulsion. Ethanol (EE) and acetone extracts (AE) (in the lowest concentration) were found to be most efficient as antioxidants in the bars. The antioxidant efficacy of these two extracts was among other related to an improved incorporation of the fish oil-in-water emulsions in the bars, high total phenolic content, high radical scavenging activity together with high interfacial affinity of phenolic compounds and probably regeneration of tocopherol. Practical applications: The work showed the application potential of F. vesiculosus extracts as a natural antioxidant in low-moisture foods such as granola bars. These findings implied that the multi-functional nature of these extracts provides not only oxidative stability of the food but also a physical stability. ejlt201500578-gra-0001 Granola bars enriched with 10% Sodium caseinate (Na-Cas) fish oil-in-water emulsion where Fucus vesiculosus extracts where added as antioxidant to control lipid oxidation. Development of secondary oxidation product, 1-penten-3-ol (ng/g granola bar) in the granola bars and confocal light scattering microscopic images of granola bar added 0.5 g acetone extract/100g emulsion (AE1) and the Control (Con) (without extract). The efficacy of acetone extracts to limit development of 1-penten-3-ol in granola bars was found to be related to extract composition as well as the microstructure, as fish oil emulsions in the bars were more stable when this extract was added. Error bars indicate SD of the measurements (n = 3).