A Research Report Submitted to Mzumbe University in Partial Fulfilment of the Requirements for the Award in Bachelor Degree of Accounting and Finance in Business Sector (BAF-BS) of Mzumbe UniversityFor a long period frauds have become a very big problem in the Banking sector.The researcher decided to study the effectiveness of internal control on frauds prevention.The main objective of the study was to measure the effectiveness of internal control on fraud prevention. A descriptive study was adopted to conduct the study at CRDB BANK PLC LUMUMBA BRANCH,a sample size of 33 out of 50 populations was adopted and structured questionnaire and interview questions were distributed to conduct study, out of 33 samples taken 28 was responded.Data collected by the researcher was analyzed by using quantitative and qualitative method,and the result was as follow Types of frauds that are likely to occur at CRDB BANK PLC Lumumba Branch researcher found that CHEQUE forgery is a common fraud at Bank , money transfer fraudsit’s a common fraud at Lumumba Branch, customer, accounts frauds also researcher found that it is a common fraud at Lumumba Branch, money laundering frauds the result found that it’s a fraud but not commonly occurred at Lumumba Branch ,and finally researcher collected data about fake payment fraud and he found that it’s not a common fraud at Lumumba Branch. Factors that contribute frauds to occur researcher found that common factors that contribute frauds are failure to engage in regular call over, poorwelfare policy in the organization, inadequate supervision of subordinate, disregards of knowing customer KYC rule,and finaly job insecurity. Impact of frauds to the Bank researcher found that frauds affect Bank to the large extent such that Bank incurs losses , also frauds lead to the collapse of the Bank , a customer also lose confidence with the Bank as frauds occur frequently. Based on the mechanisms used result found was that CRDB BANK PLC Lumumba Branch apply various procedures such as snap checks, routine supervision, checking of customer accounts opened and closed to prevent the occurrence of frauds. Effectiveness of those controls on fraud preventions researcher found that some of respondent strongly agreed and others agreed but means still there is a chance for frauds.