In this paper, carbon fibers with a high specific surface area (SSA) have been prepared from cotton for supercapacitor application. The cotton-based carbon fibers (CCFs) are prepared by carbonizing the cotton fibers in ammonia (NH₃) and nitrogen gases at different temperatures. The CCFs possess a hollow tubular structure with an outer diameter of about 7 μm and an inner diameter of about 3 μm. The hollow structure is inherited from the natural structure of the cotton fibers. The SSA and pore structure of the CCFs depend on the carbonizing temperature and atmosphere. The CCFs carbonized in NH₃ have high SSA up to 778.6 m² g⁻¹ with higher mesopore ratio. Higher nitrogen concentration (3.3 at%) and more CO functional groups are present in the CCFs carbonized in NH₃. The maximum specific capacitance of the CCFs carbonized in NH₃ is measured to be 355 F g⁻¹ at 1 A g⁻¹, 245.3 F g⁻¹ at 0.8 A g⁻¹, and 181.3 F g⁻¹ at 0.2 A g⁻¹ in KOH, H₂SO₄, and Na₂SO₄ electrolytes, respectively. The tubular structure, high SSA, higher mesopore ratio, nitrogen doping, and the presence of the oxygen functional groups are responsible for the excellent electrochemical performance. Comparing with the conventional activation process using KOH as an etchant the present process by NH₃ etching to prepare high SSA carbon materials has the advantages of simplicity, no contaminants, and higher mesopore ratio.