The mechanical properties (elastic modulus, EM: tensile strength, TS and % elongation, %E) of beta -lactoglobulin (beta -Lg) films plasticized with different plasticizers were determined. Six plasticizer types were studied over a range of concentrations. Propyleneglycol-plasticized beta -Lg films were the most brittle, with mechanical properties independent of plasticizer content. Films with other plasticizers studied (glycerol, Gly, sorbitol, Sor; polyethylene glycol, PEG 200 and PEG 400 and sucrose, Suc) exhibited negative exponential dependence on plasticizer concentration for EM and TS, while they exhibited linear dependence on plasticizer concentration for %E. The EM and TS data for each plasticizer were fitted with an exponential model, while %E data were fitted with a linear model to quantify the plasticizer effect. The EM0, TS0 and %E-0 of beta -Lg films without plasticizer determined from the fitted equations were 1500. 37.28 MPa and 0, respectively. The k(EM), k(rs) and k(E) values determined from the fitted EM, TS and %E data, respectively, reflect the efficiency of plasticizers. The k(EM), k(TS) and k(E) values indicate that plasticizer efficiency generally decreased in the order Gly, PEG 200, PEG 400. Sor and Suc. on the bases of mole plasticizer-oxygen-atom/mole beta -Lg and mass plasticizer/mass beta -Lg. These results reflect the effect of plasticizer composition. size and shape. The k(EM), k(TS) and k(E) value order was reversed when the basis was changed to mole plasticizer/mole beta -Lg. The latter results clearly reflect the effect of plasticizer number of 0 atoms.