With the purpose of collect more information about the structure of fish communities at the estuaries and mangrove forests that needed to manage and use these natural resources more effectively and sustainably. This study was conducted at the largest estuary and mangrove forest in northern Vietnam, from March 2018 to February 2019 and targeted mainly on larval and juvenile fish. We employed a larval net monthly at the midstream of Ba Lat estuary of the Red River and tidal creeks of Xuan Thuy National Park’s mangrove forest. A total of 18,509 individuals of larval and juvenile fish were collected and identified to the lowest taxon as possible. The physical features of water environment were also recorded. We recorded 78 species or type of fish that belong to 34 families/subfamilies and 14 orders. In which, numerically, Gobiopterus sp. was the most abundant, followed by Ambassis vachelli, Clupeiformes sp., Omobranchus sp., Gobidae sp.1 and Gobidae sp.2. The mean density of fish peaked in the month of May and at XL6 station, divided by time and sampling stations respectively. The number of fish species was highest in July when calculated by time, and at the station XL7 when calculated by sampling stations. The roles of mangrove forests and the estuary were discussed in the relation to distribution pattern of fish.