The choice of natural fiber purun tikus (Eleocharis dulcis) as a composite reinforcement material is related to its benefits, such as low price, abundant quantity, renewable, and environmentally friendly. In this research, the effect of alkalizing treatment and hot water immersion on the properties of purun tikus fiber were investigated. The long fiber of purun tikus measuring 20 cm and diameter < 0.064 cm was modified with 120°C hot water immersion and 5% NaOH immersion for 1 hour each. These treatments were applied to improve the properties of purun tikus fiber, such as their properties of chemical (moisture, lignin, cellulose, and hemicellulose), physical (density), and mechanical (tensile strength). The results showed a decrease in the water content, lignin, cellulose, and hemicellulose of purun tikus fiber, while the density and tensile strength increased after the treatment of 120°C hot water immersion and NaOH 5% alkalization. The NaOH 5% alkaline treatment was better than 120°C hot water immersion in terms of reducing water content, lignin, cellulose, and hemicellulose. Although the tensile strength of the modified NaOH 5% NaOH purun tikus fibers is lower than the 120°C hot water immersion treatment, it was still higher than before the modification. It is shown that these fibers blend well in the manufacture of composite materials and improve their mechanical properties.