Mangrove ecosystems are coastal areas with ecological and socioeconomic functions. This area has a function as a nutrient provider for aquatic biota. The mangrove ecosystem also plays important role in nutrient cycling in coastal areas. The process of converting nitrogen into nutrients needed by plants (ammonium (NH4+) or nitrate (NO3+)) is called nitrogen fixation. The agents that play a role in this process are bacteria. The mangrove sediment has an abundant microbial communities which responsibility to this process. Sampling was carried out in the mangrove area of Tirang Beach, Semarang using the purposive sampling method. There are three sampling points, such as in the estuary, residential areas, and ponds. The isolation process was carried out using the spread plate method on solid NFB (Nitrogen Free Bromthymol) selective media, and purification using the quadrant streak plate method on the media. There are the most bacterial colonies in mangrove locations next to settlements, range from 2-11 bacterial colonies, and the least in mangrove locations near water bodies is 6 bacterial colonies. After purification, eight pure bacterial colonies have been identified and have similar morphological characteristics, white color, circular shape, entire margin, and flat elevation.