We investigated thermoelectric power S(T) of MgB2-xBex (x=0, 0.2, 0.3, 0.4, and 0.6) S(T) decreases systematically with x, suggesting that the hole density increases. Our band calculation shows that the increase occurs in the σ band. With the hole doping, Tc decreases. Implication of this phenomenon is discussed within the BCS framework. While the Mott formula explains only the linear part of S(T) at low temperature, incorporation of electron-phonon interaction enables us to explain S(T) over wide temperature range including the anomalous behavior at high temperature.