The present work demonstrates the synthesis of one-dimensional (1D) Se nanorods with ~50 nm diameter by hydrothermal transformation of SnSe crystals in oxalic acid solution and suggests the reaction mechanism for this chemical transformation. SnSe particles react with oxalic acid to generate numerous Se nuclei, which crystallize into Se nanorods due to the intrinsic character of the 1D growth of Se. The resulting Se/SnSe composite exhibits outstanding thermoelectric power factor without the aid of any rare dopants, which is higher than both undoped polycrystalline SnSe and SnSe doped with Pb and Cu.