This dissertation consists of three essays. The first essay examines the cost efficiency and production technology in the banking sector of Thailand using a stochastic frontier approach. The empirical results indicate that banks with lower Non-performing-loanto-total-loan ratio, higher equity-to-total-asset ratio, higher liquid-asset-to-total-asset ratio, and more branches are likely to be more efficient. The second essay investigates the degree of competition in the banking industry using the new empirical industrial organization approach. The empirical results indicate that, despite the Thailand government's efforts to increase competition in the banking industry by relaxing restrictions on entry to the market after the financial crisis, the oligopolistic degree of the biggest four banks has intensified. The third essay links the results of the first and second essays, cost efficiency and competition in the banking sector, to the transmission mechanism of monetary policy in Thailand using Vector Autoregression approach. The empirical results indicate that an unexpected tightening monetary policy shock leads to higher financial costs in the banking industry, forcing banks to compete more fiercely and operate more efficiently, significantly helping strengthen the transmission of monetary policy. Hence, the policy implication is that Thai government should exert more effort to enhance efficiency and competition in the banking sector.