Added-mass (alpha) and damping coefficients (beta) of a tethered spherical float, undergoing oscillatory motion in sinusoidal waves, have been derived from the motion generated velocity potential for one degree-of-freedom (surge) using potential flow theory. Variations of added-mass and damping coefficients with respect to water depth, wave period and float size have been obtained. Variations of added-mass and damping coefficients with wave period show that these hydrodynamic coefficients are frequently dependent, and this is because motion generated velocity potential is associated with the generation of surface gravity waves by the oscillating float. The added-mass and damping coefficients are unaffectEd. by water depth, but increase with size of the float. Natural and resonant frequencies of the tethered float are computed for a wide range of wave periods using alpha and beta values. Theoretical values of natural period of sscillation of the float agree closely with the experimental values.