Processing of collagen from yellowfin tuna (Thunnus albacares) abdominal skins was optimized by response surface methodology and central composite design. The values of independent variables at optimal conditions were NaOH concentration: 0.5 N, NaOH treatment time: 36.2 hr, pepsin concentration: 1:4.9 ratio (0.245%, w/v), and digestion time: 48.1 hr, respectively. The collagen content estimated under optimal conditions was 33.1%, and the actual experimental collagen content was 32.3%. Physicochemical properties of collagen from yellowfin tuna abdominal skin were investigated by amino acids analysis, SDS-PAGE, FT-IR, viscosity and denaturation temperature. Amino acids content of the collagen was 21.0%. SDS-PAGE pattern of the collagen showed two different $alpha$-chain (${alpha}_1$- and ${alpha}_2$- chain), $eta$-component and $gamma$-component. The spectrum of FT-IR of the collagen showed wavenumber at 3,434, 1,650, 1,542 and $1,235;cm^{-1}$ representing the regions of amide A, I, II and III, respectively. Relative viscosity of the collagen decreased continuously on heating up to $32^{circ}C$, and the rate of decrease was retarded in the temperature range of $35-50^{circ}C$. Denaturation temperature (Td) of the collagen solution (0.06%, w/v) was $31^{circ}C$ and was lower than calf skin collagen ($35^{circ}C$).