A comparative study on the effect of doping of nanocarbon (C), nanodiamond, and the well-established nano-SiC in MgB2 is carried out under identical doping levels (C: 2.6 and 5.2 wt%) and processing conditions (800 degrees C, 2 h). The bulk samples are prepared via powder-in-sealed-tube method. The XRD analysis shows no reacted or unreacted phases in the case of lightly nano-C-doped sample while all other doped samples show unreacted or/and reacted phases. The a-axis shrinkage is observed for all doped samples, among which nano-C-doped samples show maximum shrinkage, an evidence for C substitution at B sites. The C substitution is also confirmed by the analysis of full-width at half-maximum values for (100), (101), (002), and (110) peaks which are broadened. The C substitution together with the presence of nonsuperconducting phases decreases the T-C to a small extent. The J(C)(H) is significantly enhanced for all doped samples compared with the pure sample among which MgB1.9C0.1 (nano-C) exhibits the best performance in J(C) enhancement up to a field of 8 T.