Geophysical flows, characterized by diverse particle -size mixtures, pose challenges for hazard risk assessment due to their anomalous fluidity and rheological behavior. This study investigates the rheological behavior of dry granular mixture flow in a flume setup using particle image velocimetry and triaxial force measurement. Key findings include a power -law relationship between the inertia value and the internal friction coefficient, affirming the applicability of the monodisperse friction model in mixed particle -size scenarios. A strong positive correlation is identified between the effective basal friction coefficient and internal friction coefficient, suggesting an influence of internal frictional properties on basal friction for granular flows on a flat surface. Additionally, positive correlations are observed between normalized stress fluctuation and both internal friction coefficient and inertia number, linking basal stress fluctuations to variations in internal frictional characteristics. These insights enhance our understanding of granular material dynamics, holding potential implications for geological disaster risk assessment.