Cellulose crystallite sizes of naturally green and brown cottons as well as of white undyed and dyed cotton were studied by using X-ray diffractometry. Data were analyzed both by a peak stripping method using the program WinFit and by whole profile matching using the FullProf program. The fit obtained with WinFit agreed with the re suits from the FullProf except for a small difference centered on the 002 reflection. Crystallite sizes of white and colored cottons were estimated by the full-width-at-half-minimum (FWHM) method, and the results showed that compared to the white cotton, the crystallite sizes of green and brown cottons based on the 101 and 002 reflections were found to be comparable, whereas smaller 10 (1) over bar crystallite sizes were observed. After dyeing, the crystallite size of dyed brown cotton displayed a slight increase in the 002 crystallite size compared to that of the undyed white cottons. The average crystallite size of 101, 10 (1) over bar, and 002 for each cotton determined by the WinFit program was comparable to that estimated by the FullProf program. (C) 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1466-1471, 2000.