Abstract C-doped MgB 2 wires have been fabricated using malic acid (C 4H 6O 5) as a carbon source through Powder-In-Tube (PIT) process using pure Fe as sheath material and sintering at 650 °C–1000 °C. The superconducting properties such as T c and H c2 increased with the sintering temperature up to 900 °C. To investigate the correlation between microstructural features and superconducting properties of the C-doped MgB 2 wires with different thermal history. TEM and EDS analyses were conducted on the specimens prepared by tripod polishing and ion milling method. Carbon particles with the size less than 5 nm were embedded in the MgB 2 grain or grain boundaries, but the distribution was not uniform. Also MgO thin layer with the thickness 10–20 nm covering MgB 2 grain was found in the sample sintered above 900 °C. Nano-sized carbon, MgO layer, and the other second phases have been characterized with high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) pattern.