Its northern and northern-western border is the river Una, which is also the state border line with Croatia, on the east side it borders with the municipality of Kozarska Dubica, and on the southwest and south with the municipality of Novi Grad. The overall surface of the municipality is 85,12 km2. There are 11 rural settlements in this area: Tavija, Petrinja, Gornja Slabinja, Kalenderi, Podoška, Zovik, Pobrđani, Mraovo Polje, Gumnjani, Mrakodol and Grdanovac. According to the 2013 census, the municipality has 5.977 inhabitants.
The Kostajnica municipality has relatively good traffic-communicational connection with other regions of Republic of Srpska and Bosnia and Herzegovina. The most important roads in this municipality are the arterial road M-14 (Bihać-Bosanska Krupa-Novi Grad-Kostajnica-Kozarska Dubica-Gradiška), the regional road R-475 (Kostajnica-Prijedor) and what is also very important, the border crossing with Croatia, which is located at the city center. The municipality also covers a part of the railway direction Zagreb-Banja Luka. It enters the borders of the municipality in the Grdanovac settlement. The distance from Kostajnica to Banja Luka is about 105 km, to Zagreb 100 km and to Prijedor 57 km.
Terrain
When it comes to its terrain the Kostajnica municipality can be divided into a hilly-knolly region, which takes ¾ of the whole territory (at the altitude from 150 to 407 meters) and it has its highest hills: Pastirevo, Balj and Um; and a range region, i.e. the river Una valley (altitude from 100 to 130 meters), which takes ¼ of the whole territory.
Climate
The area of the Kostajnica municipality has a humid continental climate. Atmospheric precipitates are evenly distributed throughout the year. In spring and autumn days thick fog is not a rare phenomenon along river Una, upstream and downstream from Kostajnica.
Hidrography
The area of the Kostajnica municipality is rich of springs of potable water, streams and small rivers. Its hydrographical net consists of river Una, its tributary rivulets of Strigova and Tavija and other smaller water flows: Mačkovac, Studena, Vučijak and Bubnjarica.