The physical properties and the structure of a poly(styrene-co-butadiene) rubber (SBR) and poly(acrylonitrile-co-butadiene) rubber (NBR) latex mixture film are studied in relation to the composition of SBR/NBR for optimization as the precursor of a polymer electrolyte. The composition of SBR50/NBR50 is most suitable in terms of mechanical strength and ionic conductivity. The relationship between the mechanical strength and the structure is analyzed using a simple equivalent mechanical model modified from the Takayanagi model. Our model gives better agreement with experimental results and extends the range of validity of the model to the cocontinuous phase type morphologies. It is possible to estimate the mechanical strength from the continuity of the mechanical supporting phase, calculated from the mechanical model. The tensile properties are found to be strongly affected by the fragile component.