The study seeks to examine the determinants of bank lending interest rates in Tanzania, largely focusing on identifying the key determinants and their relative importance. Techniques employed comprise interest rates decomposition and econometric estimation using banks' annual balance sheet data. Results on interest rates decomposition suggest that, the main drivers of lending rates are operating costs, non-performing loans; and costs of funds (deposits interest rates). The three factors accounted for 70.4 percent of small banks' average lending rates in 2014-17; while for medium and large banks, they constituted about 69.5 percent and 67.4 percent of the lending rates, respectively. Statutory minimum requirement ratio (SMR) appears to play an important role in all banks' lending rates, but its share has been declining overtime consistent with the expansionary monetary policy measures pursued since 2014. With respect to econometric estimations, the findings confirm the role of operating costs, non-performing loans, and cost of funds in explaining bank lending rates dynamics. Operating costs, cost of funds, and inflation have a statistically significant positive effect on bank lending rates, while bank size and level of liquidity have a negative influence. SMR ratio is statistically significant but bears a negative sign except for locally owned banks. In relative importance, the main determinants of bank lending rates could be ranked as follows: inflation with an average positive impact of 0.432 on lending rates for a unit change in inflation, trailed by operating costs (0.261), and cost of funds (0.255). Bank size has the largest negative effect of 0.288 for every unit increase in the variable. The implication of the findings is that effort should be directed at improving operational efficiency aiming at reducing banks operating costs. The key areas of attention are with respect to employees' salaries and benefits, as well as rental and depreciation expenses related to premises and equipment. Banks may consider to take advantage of ICT advancement in the country to cut on costs of "mortal and brick" and employees. Priority could be put on utilizing the growing agent banking framework, and digital How to cite this paper: Mbowe, W. E.,