The mangrove ecosystem of Morosari Village, located in Sayung District, Demak Regency, Central Java, Indonesia, frequent experienced water pollution problems such as tidal flooding, abrasion and pollution due to anthropogenic activities. These problems can lead to further deterioration of water quality. This study aims to use epipelic diatoms to determine the ecological status of waters based on the diatom index in the coastal waters of Morosari Village, Central Java, Indonesia. Sampling was conducted at 3 sampling locations in July-December 2022. Samples from three stations were analyzed based on the diatom index using OMNIDIA software version 6.1.5 and RStudio Software for water quality. Four diatom water quality indices (IBD, IPS, IDG, and TDI index) were obtained that could provide a percentage value of more than 70% of diatom species. The results of the analysis showed varied ecological status and water quality. Station 1 (IBD, IDG, IPS and TDI index) ranges from 10.9 to 5.2 which is categorized meso-eutrophic to eutrophic in terms of diatom species that dominate Fallacia pygmaea and Nitzschia clausii, station 2 (IBD, IDG, IPS and TDI index) ranges from 14.3 to 6.9 which is categorized mesotrophic to eutrophic in terms of species Diatoma vulgaris and Thalassiosira pseudonana, and station 3 (IBD, IDG, IPS and TDI index) ranges from 13.9 to 3.5 with diatom species dominating Cocconeis placentula and Nitzschia palea.