This study examined the sustainability of small farmer groups in extension, in Sumbawanga District, Tanzania. The specific objectives were to examine extension activities done by farmer groups; identifying factors influencing sustainability of SFGs and challenges faced in sustaining those groups. The study involved 80 respondents from two project villages and employed a cross-sectional design. Data were collected using interview schedule, focus group discussion (FGDs) and key informants interviews. The data were analyzed using content analysis and SPSSs computer programme mainly frequencies and percentages to describe the major variables. in the case of extension activities of the groups the study revealed that farmer groups were performing extension activities including demonstration of technologies in ox ripping and weeding integrated with the use of legume cover crops (87.5%) and distribution of improved inputs. Other extension activities that can be done by SFGs includes providing market information; education to livestock keepers on improved practices; and livestock dipping services. Regarding sustainability of the groups, the findings show that low participation of members’ in group activities; perceived trend of membership; activeness of members in group activities after donor withdrawal have negatively influenced sustainability of the groups. The groups were still dependent and their sustainability is doubtful. The groups were faced with challenges of availability of donors; committed group leadership; poor performance of the previous cooperatives and unreliable rainfall that made some group projects fail. This study recommended that to increase sustainability of the groups, donors should put emphasis on empowering the groups with skills and knowledge that enable them implement project activities with minimum external dependency, encourage self- reliance and involve village authorities in monitoring group activities after projectiii termination. Collective efforts among village authorities, councilors and Members of Parliament should be requested to help in soliciting funds for continuing implementing projects activities after donor withdrawal.