Micro- and nanosized, long fibers of heterobimetallic complexes, consisting of macrocyclic oxamide complexes (CuL) and transition-metal ions (Mn), have been successfully synthesized by a simple solvothermal approach. The as-prepared products were characterized by scanning electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, energy dispersive X-ray analysis, and magnetic property analysis. The diameter of the fibers can be tuned by changing the proportion of solvent. A possible microsphere to fiber transformation mechanism has been proposed to explain the formation of the products. The thermal properties and magnetic properties of the fibers were investigated, they exhibit obvious size-dependent effects. It is important to note that this solvothermal process is a general approach that can be extended to the fabrication of a series of ultra-long complex nanofibers consisting of different transition-metals.