The topic of this graduation thesis is the impact of the the war in Bosnia and Herzegovina on the ethnic structure of Bosanska Posavina. The aim of the work was to show the changes of the total number of inhabitants and the number of the three largest nations in Bosanska Posavina. Also, the goal was to show the impact that the war in Bosnia and Herzegovina had on the ethnic structure itself in the area of Bosanska Posavina. Specifically, the area that made up the Croatian Community of Bosanska Posavina was studied. The data were analyzed, processed and graphically presented by using Microsoft Office Excel. The main problem with the study of the changes of the ethnic structure were different methodologies of the population census, where until the second Yugoslavia there was no declaration on a national basis. Despite this, links between Catholics and Croats, Orthodox and Serbs, and Muslims and Bosniaks have been established by analogy. Ever since the first comprehensive population census in 1910, the Croats as a nation were the most numerous until the last census before the war, the one from 1991. The population growth in Bosanska Posavina was quite high until the middle of the second half of the 20th century, when the growth decreased. With the arrival of the 90s and the war, a major turning point occurred in the ethnic structure of Bosanska Posavina. After the war, the largest nation became the Serbs, whose number decreased slightly, while the number of Croats decreased by more than 50 percent. Unfortunately, due to the absence of the 2001 census, the 1991 and 2013 censuses are compared. Despite this, a large decline in the non-Serb, especially Croatian, population can be observed. Such devastated, divided and destroyed Posavina, which lost a third of its pre-war population, is today in an extremely difficult political and economic situation. Its future is tied to the Republic of Croatia despite the fact that it is an integral part of neighboring Bosnia and Herzegovina.