Under-five mortality in Nigeria has been reported to be on the decline, but the dynamics are yet to receiveadequate attention. Thus the main objective of this study was to assess these factors and quantify their relativecontributions to under-five mortality between 1990 and 2008. The Nigeria Demographic and HealthSurvey data for 1990, 2003 and 2008 were re-analysed to assess the trends in determinants of under-fivemortality.Cox Regression model was applied to determine the relative contributions of each factor to theunder-five mortality risk.The results showed there were improvements in maternal education (8.6%), childhoodvaccination (17.7%), use of oral rehydration therapy (13.9%) and medical treatment of childhood illnesses(17.5%) over the 19-year period. There were declines in proportions with birth interval less than 24months (3.9%), access to improved sources of drinking water (24.2%), improved toilet facilities (9.0%)antenatal care (4.5%), skilled delivery (3.0%) while maternal age at childbirth remained unchanged. Thesefactors increased the death hazards by 4.6% between 1990-2003 but decreased them by 12% between2003 and 2008. It was concluded that Nigeria has recorded very minimal improvements in birth spacing andantenatal/delivery care. Poor access to potable drinking water and sewage disposal, and short birth intervals,are among the factors fuelling childhood mortality risks. Further improvements in these environmental andhealth practices as well as other factors are recommended as strategies for promoting child survival inNigeria.