This paper analyzes the impact of the yarn linear density used in the dimensional stability of the 1x1 RIB knitwear made on the same circular knitting machine. The used samples of raw knitwear are made from 100% cotton yarn with different liner densities of 19, 17, 15 and 13 tex. Dimensional stability of these samples was analyzed by FAST 4. The results show that the most stable knitwear is made of 100 % cotton yarn with the linear density 19 tex.The values of the tightness factor in the dry relaxation stood at 14, 93, 15, 14under wet,16,32 under total and 16,20 under air conditioned conditions. The knitwear with the highest values of dimensional instability is a raw knitted fabric made of 100% CO, and the yarn with the linear density of 13 tex. The values of the tightness factor in the dry relaxation stood at 16,20 under wet, 15,14 under total and 12,53 under air conditioned conditions.