A new single-dynamical-scalar-field model of dark energy is proposed , in which either higher derivative terms nor structures of extra dimension are needed .
With the help of a fixed background vector field , the parameter for the effective equation of state of dark energy may cross w = -1 in the evolution of the universe .
After suitable choice of the potential , the crossing w = -1 and transition from decelerating to accelerating occur at z \approx 0.2 and z \approx 1.7 , respectively .