Optical switch is one of the most important components in optical communication systems. In this paper, a new photonic crystal (PhC) directional coupler switch based on slow light in coupled-cavity waveguides is proposed. Slow light phenomenon is usually applied in nonlinear optics and can be estimated by slope of dispersion curve of the PhC waveguides. By introducing this structure, the group velocity of light at the wavelength of 1550nm is obtained as low as 36 times slower than the velocity of light in vacuum. As a consequence, a short switching length of 13.6 μm is achieved for a refractive index change of 0.02 in the coupling region. The device is simulated by finite-difference time-domain (FDTD) method and the results show that the available bandwidth of the switch is approximately 9nm . Maximum transmission of 80% is also achieved in both linear and nonlinear cases at cross and bar ports, respectively.