Three lint cotton grades; Fully good (FG), Good (G), Fully good faire (FGF) were taken from each of four Egyptian cotton varieties namely Giza95, Giza98, Giza94 and Extra-long Giza96 to investigate the effect of cotton fineness, maturity and trash content on neps formation in raw cotton and yarns. In addition, studying the relationship of trash content, fiber and yarn properties. Results showed that there were highly significant effects of varieties and lint grades on most fiber and yarn properties. Giza96 gave higher fiber and yarn properties such as micronaire value, upper half mean length, uniformity index, short fiber content, fiber strength, lea count strength product, yarn strength and yarn imperfections than Giza94 cotton variety. The lint grade Fully Good gave the best values of all studied traits. Also, it had the lowest trash content, number of neps , unevenness and yarn imperfections than the Fully good fair. The interactions between cotton varieties and lint grades were significant for all studied traits except uniformity index, micronaire, maturity and yarn imperfections (yarn count 40sand 3.6 twist multiplier). Upper half mean length, micronaire value and maturity ratio had negative and highly significant correlation with trash content .While, Short fiber content and trash area recorded positive and highly significant correlations with trash content. Neps / gram and number of neps/400m recorded positive and highly significant correlations with each trash contents, yarn unevenness, thin and thick places.