Efficient banking system reflects a sound intermediation process and hence the banks' due contribution to economic growth. If commercial banks are functioning efficiently, monetary policies are likely to be effective. This study is motivated by the fact that, though banking sector is the largest part of the financial system in Tanzania, little is known about its efficiency status. Secondary time series data are used in empirical analysis of banks' efficiency. Non-parametric Data Envelopment Analysis (DEA) model is utilised in estimation of technical and scale efficiency, while x-inefficiency is estimated using multi-product translog cost function. Though banks were not full efficient in all respects, they performed fairly well during the 1998-2004 period. Nevertheless, the major conclusions show that banks still have the reasons to improve their performance.