A field experiment was conducted during 2011 Than Uyen district, Lai Chau province to determine the effects of different fertilizer levels and plant densities on yield and qualities of three improved pastures: Ghine Mombasa, Mulator II and Stylos Ubon that were selected as field experiment have potential yield and adapt to acid, infertile soil in Northern mountainous of Vietnam. The experimental design was a randomized complete block design with 4 fertilizer rates and 3 plant densities and as split-split plot treatments. Results showed that among all combination treatments, fertilizer rate and density interaction was optimal is 200 N + 100 P2O5 + 100 K2O kg /ha with 66,660 plants /ha for Ghine Mombasa, Mulator II
and 70 N + 100 P2O5 + 100 K2O kg /ha with 66,660 plants /ha for Stylos Ubon. Increasing planting density increased fiber, NDF, ADF content but had no significant effect on biomass yields. The plant densities also significantly affected protein content of pastures: protein content decreased by increasing in plant density. No further improvement in quality was obtained by increasing fertilizer. Three improved pastures were introduced for producing high quality feeds have had good impacts on increasing feed resources for ruminants, contributing to increase of number of animals raising in fanns and indirect effect on household incomes of fanners in Than Uyen districts, Lai Chau province.