The mechanism of stretch mercerization of cotton fibers was developed by a series of closely set rollers by which cotton fibers in either roving or sliver forms were mercerized and stretched to their original length of the fiber and then neutralized in the stretched configuration. The effect of stretch mercerization in the roving form on the physical properties of fibers at different stretch ratios and grip lengths was investigated. The result show that the zone stretching device developed in this study with consists of a series of set rollers and idlers provides a reasonable device for stretching the individual fibers to their original length from the shrunken state caused by slack mercerization. It also proved that the stretch mercerization in the roving form has an advantage of achieving the uniform stretching of individual fibers compared to yarn of fabric mercerization. It showed that the fibers in the robing form can undergo a more uniform stretching as compared to those in a twisted configuration. The fibers stretched to 36% at the grip length equivalent to the mean fiber length gave the maximum improvements in the physical properties of the mercerized cotton fibers.