Multilayer film composed of a soy protein isolate (SPI) inner layer and poly(lactide) (PLA) outer layers were prepared by a simple solvent casting method in order to exploit the advantageous properties of both film materials. Tensile strength and elongation at break of the multilayer film were 17.0 +/- 0.3 MPa and 176.9 +/- 27.9%, respectively. Especially the tensile strength of the multilayer film increased more than 5-fold compared with that of the SPI film. The mechanical properties of the multilayer film were comparable to those of low-density polyethylene (LDPE) or high-density polyethylene (HDPE) films. The lamination of PLA layers on SPI film also resulted in desirable gas barrier properties of the film with both low water vapor permeability (WVP) of PLA and low oxygen permeability (OP) of SPI. The WVP of the multilayer film [(6.66 +/- 0.27) x 10(-14) kg(.)m/m(2.)s(.)Pa] decreased 40-fold compared with that of the SPI film, and the OP of the multilayer film [(2.40 +/- 0.24) x 10(-18) m(3.)m/m(2.)s(.)Pa] decreased more than 26-fold compared with that of the PLA film. In addition, the multilayer film had adequate water resistance over short periods. All of these property improvements may be attributed to the strong adhesion between both polymers used, i.e., SPI and PLA.