It is quite important for considering tectonics of the terrestrial planets to study about rheology of the mineral. Rheological structure of terrestrial planets has been inferred by extrapolating of power-law to temperature and pressure of planet's interior (e.g., Mackwell et al. 1998). However, it is not sure that power-law that was constrained by high temperature experiments can be applied for Moho condition. In our study, strength contrast between plagioclase and olivine was studied utilizing two-phase deformation experiments to directly decide strength contrast between crust and mantle. Our results indicate that olivine can be weaker than plagioclase at relatively low temperature. Moreover, it is considered from our study that the strength contrast between lower crust and upper mantle is large in Venus, and there is possibility that the decoupling occur.