The synthesis and characterization of a carbon (C) doped polycrystalline MgB2 superconductor is reported with tartaric acid (C4H6O6) used as the C source. The amount of C4H6O6 is varied between 5 and 30 wt%. Relationships between microstructures, critical current density (J(c)), critical temperature (T-c), upper critical field (H-c2), and irreversibility field (H-irr) for MgB2 doped with 0-30 wt% C4H6O6 are systematically studied. A reduction in T-c from 37.65 to 34.45 K and in lattice parameter a due to the C substitution occurs with C4H6O6 doping. J(c), H-c2, and H-irr are significantly enhanced with an increasing amount of C4H6O6. All the samples exhibit a Jc above 10(4) A cm(-2) at 5 K and 8 T. This value is higher than for un- doped MgB2 by a factor of 6. The significant improvement in the superconducting properties is attributed to the lattice distortion due to the C substitution for boron, with the C coming from the C4H6O6. These findings suggest that C4H6O6 is a promising C source for MgB2 with excellent J(c) properties under high field.