To survive and succeed in competitive business environment, attracting new customers and retaining existing customers have no alternatives to a financial institution. And, in search of competitive advantages, companies are placing more and more focus on service quality. It has been observed from different researches that service quality contributes to the profitability and productivity of an organization significantly. Service companies like financial institutions can maintain successful relationship with customers if they can maintain quality of services expected by customers and keep the gap between expectation and perception of services minimal. The present study is undertaken to evaluate service quality of a leading private commercial bank in Bangladesh and measure the gap between expectation and perception of customers about financial services offered by it. To measure the quality, SERVQUAL model has been used as a tool with five different dimensions and twenty two statements. A survey has been conducted to evaluate quality and measure the gap of service quality. The management of the bank can get the insights of their service quality based on the scores of different dimensions and may take necessary measures to overcome the weak areas. The study reveals that there are gaps in every dimension of quality of services provided by the bank and the notable gaps lie with assurance, reliability and responsiveness of service quality. So, the bank should take necessary measures immediately in these specific dimensions to enhance quality of services for its sustainability