A shift control system working with a power-assisted bicycle that has an assisting-power supplying system and a derailleur. The shift control system includes a microcomputer in which plural shifting condition tables are stored. The assisting-power supplying system provides at least two different assisting modes. The shift control system also includes a shifting driver and at least one sensor for sensing at least one of a wheel speed, a crank speed, a pedaling force and a road gradient so as to generate at least one sensing signal and obtain at least one sensing result. The microcomputer selects one of the shifting condition tables in accordance with the current assisting mode for comparing the sensing result, thereby determining whether upshifting or downshifting is to be conducted. Then the microcomputer controls the shifting driver to send a shift control signal to make the derailleur upshift or downshift.