The Rungwe Cenozoic volcanic rocks, which are located at the southern end of the western branch of the East African Rift, have been provento be typical alkaline bimodal volcanic rocks by many lines of geological and geochemical evidence. The SiO2 content of Rungwe volcanic rocks exhibits a gap between 46% and 56%. Both basic and acidic volcanic rocks are enriched in light rare earth elements with highly fractionated LREEs and HREEs. In chondrite- normalized spider grams, all rocks have no Eu anomalies. The basic rocks are characterized by high abundance of Rb, Ba, Th, U, Sr, and depletion in HFSE (Zr, Hf, Ti), Nb- Ta are weakly depleted compared to La, with low Th/Ce (<0. 5) and Th/La (0. 07~0. 09) value. The incompatible elements of acidic rocks are enriched todifferent degrees, and depleted in Sr, P and Ti. Compared to La, Nb- Ta is weakly enriched suggesting stronger crystallization differentiation of magma. Pb is enriched in all the samples, indicating that there is a trace of crustal contamination. The basic rocks are characterized by ε Nd ( t )=+1. 50~+3. 70, ( 87 Sr/ 86 Sr)i=0. 7048~0. 7057, while acidic rocks are characterized by ε Nd ( t )=+0. 21~+5. 13, ( 87 Sr/ 86 Sr)i=0. 7043~0. 7061 except the T19 sample which has negative ε Nd ( t ) value. These data indicate that the acidic rocks are similar in Sr- Nd isotopes to basic rocks and that the basic rocks were derived from a depleted mantle with contaminationfromthe continental lithospheric mantle, while the associated acidic rocks were formed by fractional crystallization of the basic magma.