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The electronic properties of a nanographene ribbon can be significantly tuned by a spatially modulated electric field. The modulated electric potential results in the changes of the electronic properties, i.e., modified energy dispersions, creation of extra band-edge states, alteration of the energy gap, and induction of semiconductor-metal transition. The number of the free carrier increases with the increment of the field strength. Through further classification of the carbon atoms, the features of the wave functions are clearly presented, and the carrier distribution is drastically modulated under the influence of the electric field. The periodic length and the phase shift of the modulated electric field induce a change in the y-axis symmetry of the ribbon and have a significant influence on the energy of the partial flat bands, the energy gap and the carrier distribution. The characteristics of the band structure are directly revealed in the density of states (DOS). The number, heights, positions, and spacings of the peaks in DOS are significantly changed. At the Fermi level, DOS is considerably enhanced; that is, more free carriers are created. The predicted results can be verified by optical and transport experiments.