Sirospun spinning system, which holds two-folded yarn at one process, was promoted by some changes on ring spinning machines. From past to present various researches on Sirospun yarn production are existed and various parameters which affected yarn properties were analyzed. On this study, yarn properties of Sirospun blended yarns were investigated. The purpose of this work is to examine the influence of raw material, yarn count, twist multiplier and strand spacing on the yarn properties of %100 polyester and polyester/cotton blend yarns. To this end, 100% cotton, 67% cotton / 33% polyester, 33% cotton / 67% polyester and 100% polyester Sirospun yarns were produced and physical properties of the yarns were measured. Furthermore, ring spun yarns were produced for comparison with same raw materials. With the analyses of variance implement, results were evaluated and the influences of raw material, yarn count, twist multiplier and strand spacing on the yarn properties were analyzed. After the analyses, effect of the raw material, yarn count, twist multiplier and strand spacing on the yarn properties was found statistically significant (p<0,05).