The objective of this work is to determine the effect of alkaline treatment parameters on IFSS of sugar palm fiber-polyester system. Whilst the fiber was obtained from local sources, the polyester was produced by the PT Justus Kimia Raya. The parameters being studied are alkali content of the solution (0.0, 2.5, 5.0 dan 7.5 wt%), and soaking time (0, 2, 4, da 6 jam). The experiment was carried out in two stages. First, the soaking time was kept constant at 2 hrs, and the alkali content was varied. The alkali content producing the highest IFSS was then used in the second stage where the soaking time was varied. Fiber surface morphology was determine by closely observed the SEM photo micrograph of the the samples. Specimens were treated sugar palm fibers embedded at various depths in two blocks of polyester (one at each end), then axially loaded. The IFSS was obtained from those fibers who were pulled out. Cross-sectional area of the fiber were determine by beasuring their respective SEM photo micrograpgh of the samples by means of an open source software, the imageJ. It was revealed that the highest IFSS being 5.24 MPa for large fibers, and 5.04 MPa for small fibers was obtained at 2.5 wt% alkali content and 2 hrs soaking time. At higher alkali content and longer soaking time, the IFSS was found being lower that may be caused by deteriaration of cellulose microfibril at the outer layer.