Applying the Data envelopment Analysis (DEA), this paper, first, estimated technical efficiency (TE), pure technical efficiency (PTE), and the scale efficiency (SE) of the Islamic banks of Bangladesh during 2008-2012. The DEA results found that the average TE, PTE, and SE score ranged between 98.4 percent and 99.5 percent, 99.6 percent and 99.7 percent, and between 98.4 percent and 99.8 percent respectively. Second, the paper applied Tobit for estimating the significant factors determining the efficiency of the Islamic banks of Bangladesh during 2008-2012.The Tobit results showed that the efficiency of Islamic bank was positively related to the capital adequacy (EQTA), the number of bank branches (BRANCH), and negatively related to the poor loan quality (LLTA), higher liquidity claim (DEPLOAN), and the bank size (BKSIZE).