Some methods for estimating the over-all oxygen transfer coefficient (K_Lα) in surface aeration are investigated. The dissolved oxygen in water is removed with nitrogen gas or sodium sulfite. When nitrogen gas is used, the moment method reproduces C_L-t curve so exactly that it is thought to be the best method to estimate K_Lα in this example, and the log value difference method follows. However, the moment method is not suitable when dissolved oxygen is removed with sodium sulfite, because the K_Lα value estimation is easily affected by the existence of residual sodium sulfite. Since the log value difference method does not have such an inconvenience, it can be applied more widely.