Two carboxylation processes were carried out to modify multi-walled carbon nanotube bundles (m-MWNTs). The results showed that both of the m-MWNTs could be highly dispersed in water with the concentration up to 25 and 15mg/mL in ethanol without obvious deposition or conglomeration over 6 months. FTIR, Raman, TGA and TEM characterizations of the mMWNTs showed that hydroxyl and carboxyl groups have been attached with MWNTs, and that the nanotube structure was only damaged a little after modification treatments in mixed acids over 24h. Above prepared nanotube suspensions in water were successfully used as a filler of thermoplastic PVA composite to enhance its electrical conductivity. The results showed that the addition of m-MWNTs greatly improved the conductivity and the threshold of mass fraction was about 5wt%. The results also proved that purification process was not needed because of high purity of the raw MWNTs prepared in our group.