We found stereoselective thermal isomerizations at the phosphorous centers of triphosphasumanenes in the solid state. Theoretical calculations and thermal analysis indicate that the isomerization is dominated by the thermodynamic stabilities of crystals depending on the alkoxy groups on their skeleton. The stereoselective isomerization of triphosphasumanene was utilized for the synthesis of a high out-of-plane anisotropic synderivative from a less out-of-plane anisotropic anti-derivative.