A series of C doped and undopedMgB2 tapessintered at 600–950 °C was fabricated. The relationships between the critical current properties,crystallinity and microstructures were systematically studied as functionsof the C doping level and heating conditions. A decrease in thea-axis length due to carbon substitution was observed in the Cdoped samples. The full width at half maximum (FWHM) of theMgB2 (101) peak increased with increasing C doping level, suggesting that theintroduction of lattice defects and/or the decrease of the grain size ofMgB2 occurred on C doping. Due to the substitution of C for B, the upper critical field was improved in Cdoped MgB2 samples. Substantially improved critical current density,Jc, was found in all of the C doped samples. At 4.2 K, 10 T,Jc reached 2.2 × 104 A cm−2 in the 8% C doped samples.