Generally, natural fiber consists of three main components that bind to one another to form a bundle of fibers, namely lignin, hemicellulose, and cellulose. Lignin and hemicellulose have an amorphous structure, and cellulose has a semicrystalline structure. The mechanical properties of natural fibers depend on the amount of crystalline structure in the fiber and for this case is crystalline cellulose content. One method for separating crystalline cellulose in fiber is by alkali treatment. These papers describe the effect of time alkali treatment on chemical composition change and tensile properties of kenaf fibers. The fibers were immersed in 6% wt. sodium hydroxide solution with different time (1,3,8, 24 h) at room temperature, rinsed, and dried. Samples were resulted from the experiment will be characterized by tensile strength, Fourier Transformation Infra-Red (FTIR) Spectroscopy, and Field-Emission Scanning Electron microscope (FE-SEM) instruments. This study shows that the tensile strength properties of kenaf single fiber with alkali treatment in 6% wt. sodium hydroxide at room temperature at different times, increased. The maximum tensile strength was resulted from alkali treatment in 6% wt. sodium hydroxide at room temperature for 8 hours of 553.3 MPa. It was considered that improvement of tensile strength upon alkalinization was correlated with change the chemical structure and morphology of the fibers.