In this paper, the authors investigate viscous effects on the characteristics of two-dimensional hydrofoils. The pressure distribution on a foil section in non-cavitating flow should be considered before its cavitation characteristics study. Theoretical results which neglect viscous effects do not always agree with experimental ones, especially at a high angle of attack or at thin hydrofoils. Moreover, recently Arakeri et al. reported that laminar separation bubble played an important roll on the cavitation inception. From these considerations, it is very important to study on the viscous effects of the foil sections-specially on the mechanism of separation bubble. Separation bubble is classified into two types. One is called “short bubble” which can reattach to hydrofoil surface (non-burst type). The other is called “long-bubble” which can't reattach to the foil (burst type). The authors investigated their nature and mechanism in detail, and verified the effects of separation bubbles on the hydrofoil characteristics. An empirical classification method of the separation was proposed, which can be used to evaluate the viscous effect of any hydrofoil sections. Then, experiments were performed to support this empirical method on 5 hydrofoil sections. A good agreement between empirical method and experiments was obtained.