The effects of blending ratio(P/C) and yarn count(Ne) on the stress relaxation behaviors of polyester/cotton blending air-jet spun yarn(MJS yarns) compared with those of ring spun yarns were examined. And for the stress relaxation of MJS yarns the conclusions obtained from this study are as follows ; 1. Stress relaxation behaviors of MJS yarns were affected by the reinforcing effects of strength due to polyester fibers and by the reel ability of wrapping fibers in the constant blending ratio and yarn count. 2. Specific reaction rate, K, was increased with the larger amounts of polyester fibers and the finer yarn count. 3. The size of initial relaxation site, $alpha$$_1$, was decreased with the larger amounts of polyester fibers and the finer yarn count whereas that of later relaxation site, $alpha$$_2$, was increased on the same condition of $alpha$$_1$.