The term of colour fastness is usually associated with colour and is of vital importance to all sectors of the textile industry, including production, distribution, and most importantly, the consumers of textiles. Colour fastness, especially in the precious textiles, such as hand-made carpets, are important. Since the sample preparation for the colour fastness assessment of hand-made carpet in conventional way is destructive, a non-destructive method should be employed. A proposed non-destructive method involves taking micro-sample from the yarn of carpets. As the size of these samples are small, therefore the size effect on colour appearance and consequently on colour fasstness of the samples should be considered. In the present research the effect of sample size on the colour fastness assessment is investigated. To do so, 28 simulated colour fastness panels were prepared by the available dyed cotton fabrics. Then, by locating a mid-gray frame on the panels, five visual fields of 1 o , 2 o , 4 o , 6 o and 10 o were obtained and the colour fastness of the panels was assessed visually by using ISO 105-A02 standard method. The normal vision of the observers were examined by Ishihara test. Then, by the application of Xiao’s colour size model and by applying instrumental colour fastness formulas, GRC and ISO, for the five sizes the colour fastnesses were calculated and compared with the corresponding visual assessment. The results showed that the visually assessed colour fastnesses are higher than the corresponding calculated colour fastnesses. Also, it was shown that the visually assessed colour fastness is affected by the sample sizes, i.e. the bigger is the size of the panels, the lower is the reported visual colour fastness. This trend is the same for the calculated colour fastnesses but with much less intensity. Keywords: Size effect, Colour fastness, Visual assessment, Instrumental assessment.