A facile method to modify the surface of multiwalled carbon nanotubes (MWCNTs) via electrostatic interactions between polyelectrolytes and oxidized MWCNTs was developed. Diblock copolymers containing poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC), a positively charged block, and poly(ethylene glycol) methacrylate (PEGMA), a neutral block, with tailored molecular weights and low polydispersities were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. Acid treated-MWCNTs were coated with the RAFT diblock copolymers to improve their dispersibility in aqueous phosphate buffered saline (PBS) solution. The short positively charged PMETAC block was designed to attach the block copolymers to the surface of MWCNTs via electrostatic interactions, whereas the PEGMA block improved dispersibility of the MWCNTs in aqueous solutions. Extensive screening of the diblock copolymers with different degrees of polymerization (DP) showed that the dispersion stability of the polymer-coated MWCNTs in PBS was greatly improved with increasing chain length of the PEGMA block. In particular, the MWCNTs coated with a diblock copolymer containing PEGMA (DP = 118, the longest block investigated) showed superior dispersion stability in both water and PBS solution.