Most recent studies on polymer nanocomposites containing carbon nanotubes include mechanical, rheological, thermal and electrical properties. Optical properties of polymer nanocomposites containing carbon nanotubes are not very clear. Accordingly, in the present work, the optical properties of these naocomposites are studied in the visible region of electromagnetic spectrum. To do that, PMMA nanocomposites including different percentages of carbon nanotubes (CNTs) and carbon black are prepared by applying melt mixing method. To characterize the optical properties, the spectral reflectance values of the prepared samples are measured by a reflectance spectrophotometer with d/8 geometry in the visible region from 400-700 nm. The color parametric values a*, b* and L* of the samples are calculated in CIELAB color space under a simulator of the day light i.e., D65 and the observer 1964 (10o). For all forms of samples, i.e. powder and different prepared nanocomposites, the blackness index, proposed by Westland and et al., is calculated. For the latter, the transmission electronic microscope (TEM) images are obtained, also. As a result, with increasing the nanotube content up to 0.5 %wt, the spectral reflectance factors decreases down to 7% and the blackness content of the samples increases. In addition, the results show in comparison with carbon black, the samples containing carbon nanotubes represent a better blackness index. The TEM images show that for specific carbon nanotube content, the spreading CNT by melt mixing method is better than solution method. It seems that CNT can replace carbon black by its more efficient distribution by an appropriate method such as melt mixing.