When cotton fibers are blended with synthetic fibers, a great deal of compatibility must be achieved to realize the benefits of this fiber mixture. This compatibility stems from three basic interaction modes: structural compatibility, attributive compatibility, and appearance compatibility. These modes collectively make up for a homogenous and predictable fiber mixture. In this paper, the focus will be on characteristic compatibility. Of particular emphasis, a key question will be how fibers of different components assist each others in meeting the desired performance of yarn. In this regard, critical attributes such as fiber dimensions, bulk density, and fiber strength will be analyzed. In addition, the concept of length utilization efficiency presented by El Mogahzy and Kirifa will be implemented for the mixtures of this study. Fiber blends considered will include: cotton/polyester and cotton/micro-denier modal fibers.