Research has been conducted in May - June 2019 on the Rangsang Island.This study aims to determine the effect of mangrove forests to coastlines contained in Rangsang Island Riau Province. Sampling location determined by purposive sampling that is by 3 station points that’s Segomeng village, Sungai Gayung Kiri village, and Tanjung Kedabu village. Then analyzed at the Laboratory of Physica…