The effects of phlorotannin extracts (PTE) (from sporophyll of Undaria pinnatifida) added at different concentration (0, 1.6, 8, and 40 mg/g protein) with and without UVA irradiation treatment on gel properties of mackerel (Scomberomorus niphonius) mince (MM) were investigated. The results indicated that both the breaking force and water holding capacity (WHC) of the MM gel increased with addition of PTE, and the highest breaking force and WHC were observed with 40 mg/g PTE under UVA irradiation. Low-field NMR revealed that free water significantly decreased in this PTE-MM gel as well. UVA irradiation treatment led to reduction of total sulphydryls and free amines in the PTE-MM gel, indicating more cross-linking between proteins. Electrophoresis results indicated that myosin heavy chains (MHC) were aggregated in the UVA-treated PTE-MM gel. The microstructure of the UVA-treated PTE-MM gel showed that protein formed more evenly distributed cross-linked structures and was more compact than the untreated control gel. As a conclusion, PTE addition with UVA irradiation is shown to improve gelling properties of mackerel mince. Polyphenols are known to react with amine groups on the side chains of peptides under oxidative conditions, leading to the formation of protein cross-links. UVA irradiation promotes the formation of free radicals, which could generate strong oxidative environment. UVA plus polyphenols hence could enhance protein cross-linking which is critical in the manufacture of fish mince gel products. Phlorotannin extracts (PTE) (from sporophyll of Undaria pinnatifida) is rich in polyphenols. With UVA induced oxidation, PTE could be used to improve fish mince gel properties. The purpose of this study was to investigate the protein structural changes of UVA-PTE-aided mackerel mince (MM) gels, and monitored their texture, WHC and other physicochemical characteristics to provide theoretical foundation for industrial product development. This article is protected by copyright. All rights reserved.