Abstract: The objective of this study was to evaluate fiber quality characteristics of Naeini sheep. An experiment was carried out on six flocks from different regions of Isfahan province. Two hundred and twenty seven Naeini sheep (191 ewes and 36 rams) were sampled from a 100 cm2 surface on the mid-side. Fleeces shorn in 1999 within two sampling seasons (June and December) were sent to the laboratory of Animal Sciences at Isfahan University of Technology in order to study some wool quality traits including: fiber diameter mean, variability in fiber diameter, percent of true, modulated and heterotype fibers, accumulation of scales in 100 μm fiber lengths and diameter mean of modulated and heterotype fibers on Naeini sheep. Fiber diameter mean, within sample variability of fiber diameter and diameter mean of heterotype and modulated fibers were estimated 28.51, 11.19, 46.67 and 65.78 μm, respectively. The percentages of true, modulated and heterotype fibers were 89.53, 5.93 and 4.38, respectively and the average number of scales was 6.2 per 100 μm length of fiber. Herd and season had significant effects on all of the studied measures. Herd effects may be due to genetic, environment and pasture (nutritional) differences between herds. Also, the results of this study showed that there would be finer wool type at June than December shearing times for Naeini sheep. Age had a significant effect on % of heterotype fibers only and sex effect was only significant on the scales accumulation. On average, Naeini rams had finer fibers than ewes, which indicate higher selection intensities on rams. The results of this study revealed that Naeini sheep has great qualifications for carpet industry. However, further breeding programs are needed to meet textile industries qualifications.