A mountable arm assembly for fitness equipment, comprises two arm members and a connector for pivot connection of the two arm members. The two arm members respectively provide a plurality of assembly portions for length adjustment of the tension arm. The connector is formed with a pivot connection portion and a plurality of eccentric positioning portions. Specifically, an adjustable disposing angle is formed between the two arm members, and the two arm members set the disposing angle through the one the eccentric positioning portions. The arm assembly can be mounted on fitness equipment and can swing back and forth through the pivot connection portion. The users can select one assembly portion on each of the two arm members for required tension arm lengths and add an applied force and a resistance force respectively on the two assembly portions.