Multi - walled carbon nanotube (MWNT)/poly (methyl methacrylate) composites were fabricated with the variation of the nanotube - concentration through solution casting. It was confirmed that the nanotubes were well dispersed in PMMA according to SEM images. Because MWNTs in MWNT/PMMA composites were assumed to be randomly oriented, the tensile modulus of the MWNT/PMMA composites was evaluated through the Tsai-Pagano equation that had been applied to short fiber composites for estimating their modulus. For investigating the mechanical properties of the MWNT/PMMA composites, tensile tests were performed varying the MWNT - concentrations. For each MWNT - concentrations, at least 5 specimens of MWNT/PMMA composites were made and tested. As the MWNT concentration increased from 0 to 0.15 wt%, MWNT/PMMA composites were improved by about 20% in the tensile strength and by about 32% in the tensile modulus. However, the experimental results from tensile tests were not in agreement with the estimated results. It was supposed that there were two reasons. First, MWNTs in this research were not stretched straightly but entangled ones. That is, MWNT could not be assumed to be a short fiber. Second, the concentration of MWNT is too small to be compared with that of the general short fiber composites.