The study assessed key factors that influence the usage of mobile banking services in Tanzania using TAM and transaction theory. A sample of 120 mobile phone users collected through a structured questionnaire was used. Descriptive, Pearson correlation and multiple regressions were employed for analysis. The study found that customers’ awareness and perceived ease of use have a significant positive influence while perceived risk and transaction cost have a significant negative influence on the usage of mobile banking services. Customers’ awareness and perceived ease of use have relatively greater influence on the usage of mobile banking services than perceived risk and transaction costs.