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As a most promising mid‐temperature thermoelectric material, CoSb3‐based bulk material exhibits an applicable figure‐of‐merit (ZT) of more than one. However, their fabrication is historically time‐consuming due to the long‐time solid‐state phase transitions from CoSb2 to CoSb3. To overcome this challenge, here, a fast one‐step process is developed to fabricate n‐type Yb‐doped CoSb3 with stable ZT of 1.12 at 765 K in <5 h. Experiments confirm Yb promotes peritectic reactions of CoSb + Liquid → CoSb2 and CoSb2 + Liquid → CoSb3, optimizes power factor, and suppresses thermal conductivity. Moreover, the dense grains, induced by the one‐step crystallization, result in outstanding mechanical properties with a Young's modulus of 171.4 GPa and a hardness of 8.8 GPa in the Yb‐doped CoSb3. This study indicates that the fast one‐step fabrication route can effectively promote the practical applications of CoSb3‐based thermoelectrics and provide guidance for thermoelectric fabrication via rational phase design. [ABSTRACT FROM AUTHOR]