It is well known that it is not easy to develop a polymer nanocomposite with nanocarbon materials, NCMs (carbon nanotube; CNT or graphite), which have several superior properties such as electrical conductivity, thermal conductivity and strength. The difficulty comes from the less-reactive high surface area composed of sp2 bonding. The large specific surface area of NCMs which is basically hydrophobic and interacts with the van der Waals (vdW) interaction makes the nanocarbon materials strong bundled or clustered structure. Due to defibrate or break the bundle structures in the hydrophilic polymeric matrix, we have developed a new melt mixing method under a high pressure gas situation. The polymer melts mixing is carried out under unfilled state but the unfilled space is filled with supercritical fluids (SCFs). The SCF such as CO2 makes polymers plasticized and enhances the mobility of polymer chain and also has a similar density of hydrophobic organic solvents without interfacial tension, and can penetrate into spaces between bundles. If the surface of NCMs is filled with pressurized CO2, polymer chain has much possibility to diffuse into the bundles space and the shear stress applied by the mixing machine enhances defibrate of bundles. The advantage of the high pressure fluid mixing is that it is possible to disperse NCMs into polymer matrix with less frequency of breaking. In this study, we have performed the mixing under high pressure CO2 and N2 circumstances, we found that the mixing under CO2 with enough mobility of polymer chain produced a well dispersed CNT composites with less breaking CNT. However, 2-D nanocarbon such as a graphite is not easy to disperse because of more high vdW interaction than that of CNTs.