The objective of this study is toinvestigate the flexural behavior of externally prestressed concrete beams. First, series of test specimens of PC beams with external, internal unbonded tendons and bonded PC beam were conducted. It was found that the effect of change of tendon's eccentricity (secondary effect) is dominant in flexural strength of externally PC beams. Second, the established analytical approach based on the compatibility of deformation and change of eccentricity effect showed a good agreement with test results. Finally, design equations to predict flexural strength of such an externally PC beam were proposed and the better accuracy of prediction based on this proposed equations was obtained when compared with the other investigator's proposed equations.