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In a complex nanostructured material or composite, the thermoelectric figure of merit, Z T (T) , is strongly dependent on the morphology and transport properties of interfacial areas that connect individual crystal grains or composite constituent phases. Despite the active efforts to improve Z T (T) , relatively few experimental and theoretical investigations have been focused on probing the interfacial transport properties of relevant materials. We developed a finite element method based mesoscale-level simulation approach to evaluate the effective values of thermal and electrical conductivities, and Seebeck coefficient in structurally complex materials. This approach was tested on three popular nanocrystalline thermoelectric systems: n-type Si, n-type Si 0.80 Ge 0.20 , and p-type BiSbTe, providing excellent agreement between the simulated and previously measured values of Z T (T). The interfacial thermoelectric properties of these material systems were quantified in the process and compared to simple models. Furthermore, the sensitivity of Z T (T) to changes in the system morphology was elucidated as well. [Display omitted] [ABSTRACT FROM AUTHOR]