Fiber fineness is an important factor in yam strength and uniformity properties that depend on the average number of fibers in the yarn cross section. Fiber perimeter is the variable that is of great effect on fiber tensile strength properties. New fineness equations depended on pl and ph measured by Micromat Tester were used to calculate fiber perimeter, diameter and wall thickness for some Egyptian cotton cultivars (Giza 45, Giza 88, Giza 85, Giza 86 and Giza 83). Significant correlation coefficients were found for all properties from Micromat Tester versus Image Analysis. The relation between the data from the two instruments was found to be quite strong (r = 0.9273, 0.9433 and 0.8856) for fiber perimeter, diameter and wall thickness; respectively. So the values of cotton fiber fineness (diameter and perimeter) as well as fiber maturity (wall thickness) could be determined rapidly by Micromat Tester. Success in this approach would provide an acceptable reference method, which is not currently available. The breeder and the spinner will get direct, accurate and fast determinations of biological fineness in terms of fiber perimeter or diameter and maturity in term of wall thickness.