Mercerization of cotton fabrics affects their various properties including physical properties and dyeing behavior. In this study, the concentration levels of NaOH solution, with 18% and 25%, and the mercerization temperature levels, $22^{circ}C,;10^{circ}C$, and $5^{circ}C$, were changed in order to investigate the physical properties and dyeing behavior using Gardenia, a natural dyestuff, and direct dyes. The effect of tension during the mercerization was also investigated. In order to investigate the dyeing behavior of Gardenia, a direct dyestuff was employed as a comparative material for better objective analysis and evaluation. It was found that the mercerization condition of 18% NaOH concentration at $10^{circ}C$, without tension, resulted in the highest ${Delta}E$ value, when dyed with Gardenia.