The study was carried out on station at Agricultural Research Institute- Uyole and on-farm at Mwandobela during the 2011/12 cropping season. The objective of this study was to improve crop productivity and soil fertility of smallholder farmers through use of five different leguminous cover crops. The study was carried out in two phases. The first phase was aimed at determining biological nitrogen fixation of the cover crops. The second phase was aimed at determining the rate and pattern of nitrogen release from decomposed cover crops under field condition. The field experimental design was a Latin square applying six treatments with six rows and columns. The treatments included Canavalia ensiformis, Mucuna pruriens, Vigna unguiculata, Dolichos lablab, Glycine max and Zea mays as reference crops. The decomposition experiment was conducted in a split - split plot with three replications. The main-plot factor were aboveground and underground incubation position; while the cover crops as sub-plot factor and the six sampling periods as sub–subplot. Data for field experiment were plant stand at emergence, plant height, ground coverage, days to 80% flowering, dry matter and nodulation. The results revealed that different legume cover crops have different potential of fixing N2 and accumulating dry matter. Velvet bean had high N2 fixing potential (101.9 kg N ha-1) and accumulated high dry matter (19.5 t ha-1) followed by cowpea (50.3 kg N ha-1 and 10.5 t ha-1). There was an increasing trend of soil N with time of decomposition. However, faster rates of increasing soil N were observed at 12-15 weeks of decomposition for jack bean (0.09 - 0.13 % N), velvet bean (0.08 – 0.12 % N) and cowpea (0.11 – 0.12 % N) when placed both above and underground position.Government of Tanzania through the Ministry of Agriculture Food Security and Cooperative