The ethnic and religious composition of the interwar population of Poland was largely heterogeneous. The purpose of the article is to study how advanced was the demographic transition process in Poland in 1931, and whether the level of advancement differed among various ethnic and religious groups. The ethnic diversity of the Poland's population was depicted at a level of voivodships, grouped accordingly to their population composition by ethnicity. The main demographic indicators: births, deaths, natural increase, and the rate of demographic dynamics were analysed by voivodships for the main religious denominations (Roman Catholic, Greek Catholic, Orthodox, Protestant and Jewish). The differences were found to be also related to other factors (illiteracy level, sources of income) as well as to the stage of the epidemiological transition, which influenced advances in the demographic transition. The primary statistical database came from a detailed investigation by the Central Statistical Office, published in statistical journals entitled 'Marriages, Births, and Deaths', and also contained data of the 1931 population census.