Reduction of under-jive mortality is one of the millennium development goals. Over the past tworndecades the level of inji:lI1t and child mortality has been declining in Ethiopia. Nevertheless, itrncontinues to be among the highesl in the world A substantial difference in the level of childhoodrnmortality has also been observed among regions of the country. In-depth underslanding of thernlevels, trends, differentials and determinants of childhood mortality is. therefbre. crucial in anyrnattempt to attain the goal of reducing infant and child mortalily level through any kind ofrnintervention. In this regard, this study examines the delerll1inants of infant and childhood morlalityrnin the Southern Nalions Nationalities and Peoples Regional State (.,)NNPRS) ()fEthiopia.rnThe main objective of this study is 10 assess the effects of socioeconomic. demographic, maternalrnhealth care and environmental ji:lctors on infant and child mortality. Data for the study wererndrawn from the 2005 Ethiopian Demographic and Health Survey. A tolal of), 730 children born inrnthe past five years preceding the survey were used as a unit of analysis. Cox Proportional hazardrnmodel regression analysis was used to estimate the net effects of each of the covariates included inrnthe model.rnThe study reveals that the demographic variables are important in determining child survival inrnthe region and the risk of mortalily is high for births with short preceding birth interval Â«2rnyears), short duration ofbreasljeeding Â«6 months). births to young women Â«20 years) andfirstrnand higher order births. Survival status of the previous child at the birth of the index child is alsornfound to have an effect on infant mortality. From the socioeconomic factors, maternal education,rnmarital status, toilet facility and ethnicity show slight impact in determining infant and childrnmortality in the region, Ihough their independent effect is not slatistically significant.rnPolicy focus should be on promotion of breasljeeding and improvement o/family planningrnprograms in order to increase birth interval lenglh and reduce both childhood mortality andrnfe rtility in the region.