This study examined the levels and patterns of under-five mortality in Nigeria within 2003-2013 using the Nigeria Demographic and Health Survey datasets for this period. In the study the mortality trends was related to some socioeconomic and health variables to explain factors that could have contributed to mortality changes. The descriptive approach through the use of tables and charts was adopted in the analysis. Results show that under-five mortality rate had declined from 187 in 2003 to 128 in 2013 (i.e., a decline of 32 percent). Under-five mortality declined with increase in mothersâ€™ education especially among mothers with less than secondary education. During the ten-year period, children who had DPT3 increased from 10.4 percent to 22 percent. Households who drank water from safer sources increased from 14.4 percent to 49.7 percent for urban areas and 2.3 percent to 32.7 percent in rural areas. Childhood diarrhea incidence declined by 45.2 percent. The study concluded that decline in under-five mortality may be attributed to improved immunization, safer water sources, and reduced incidence of childhood diarrhea. This study, therefore, recommended that both the national and state governments of Nigeria should intensify efforts to increase childhood immunization coverage, provide more communities and households with safe drinkable water in order to drastically reduce diarrhea diseases â€“ a major cause of deaths in children. This will help to accelerate the decline in under-five mortality rate in the country.