The temperature dependences of the thermoelectric power of a single crystal Bi2Sr2CaCu2O8 were measured in both the a and b directions. We discovered the anisotropy of thermoelectric power in the a-b plane of the crystal. Suggeste that the differences between Sa and Sb are caused by the modulate structure in the b direction. A satisfactory explanation is given by taking into account the contributions of the relatively mobile electrons Sd, the relatively localized electrons Sh and the phonon drag effect Sg.