Quasi-phase-matching (QPM) is widely used for nonlinear frequency conversion with quadratic media, in which QPM is achieved by periodically reversing the signs of the nonlinear optical coefficients. Although efficient third harmonic generation (THG) can be realized by cascading two quadratic processes, direct QPM THG with isotropic media is of fundamental interest. For an isotropic cubic medium, however, reversal of the sign of the nonlinearity is not possible. Recently, we reported the first experimental realization QPM THG in isotropic polymer films with alternatingly deposited multilayers of a highly nonlinear organic material and a passive resin up to four periods . Here, we investigated the QPM characteristics by measuring the THG output while tuning the fundamental wavelength around the QPM condition.