From the psychology point of view, gray is a compromised color, since it is neither white nor black, but a transition between these two colors. Gray applies a constant effect on the colors with which it comes in touch. In Munsell system, grays are only determined by their value as being within the close range from white to almost black. In recent years gray materials have many applications in the construction of various objects. However, few studies have been performed on colorimetric and spectral characteristics of gray samples. The aim of the present research is to find the colorimetric characteristics of grays. To do so, 484 simulated square patches measured 3*3 cm 2 , which were distributed in the achromatic and near achromatic range of color space, were shown on a mid-grey background using a characterized Lacie LCD monitor. The monitor was placed in a darkened room and the patches were presented randomly to 30 observers. The normal vision of the observer was examined using Ishihara test. The observers were asked if the presented patch is grey or not. Then, using a psychophysical method, the patches were ranked on the basis of their grayness. Next, the gamut of the grays in CIELAB color space was obtained. The impacts of the L*, C* and H* of the patches on the grayness perception were also determined. It was shown that the recognized grays in the third region of the color space, with green-blue hues, are given higher rank by the observers, however, the effect of the brightness and chroma variation on the gray perception should also be considered. This is in accordance with the results of the higher perception of whiteness and blackness for the blue-tinted white and black samples, respectively. In addition, to examine the effect of the background of the patches on their grayness perception, two more backgrounds with L* equal 40 and 60 were chosen and the experiment was repeated. It was concluded that due to the crispening and simultaneous contrast, the gamut of the grays are changed, in a way that the gamut of grays is increased in the near white samples for the background with higher lightness, however, the similar effect for the background with lower lightness and near the black samples was not observed. Keywords: gray, color range, background brightness effect, ranking