An investigation on the dimensional properties of plain knitted fabric produced from cotton yarn and subjected to different relaxation treatments is presented in this paper. The main aim of this research is to characterize the internal energy of fabric by using yarn-pullout test method in ultrasonic relaxation state and compare it with other common mechanical relaxation treatments. A comprehensive experimental analysis showed that, by using ultrasonic waves, the area geometry constant value $(k_s)$ achieved was higher than the conventional relaxation methods. Then, we introduced residual-energy forming and loss-energy uniforming. The former appeared due to fabric shrinkage and the later one appeared due to release of fabric loops from extra forces which imposed during knitting process. The results also indicated that ultrasonic waves energy causes more uniformity on loop and consequently, the fabric reaches to a less internal energy than washing treatment.