We report on the GaAs asymmetric X-junction waveguide coupler. The mechanism of this coupler is discussed. The device is made on n-/n+ GaAs epitaxial material. The relation among the mode sorting angle and waveguide ridge height and the difference between the propagating constants of two asymmetric waveguides is analyzed by using an effective-index approximation. The device is characterized by a power division of -2.9 dB and a crosstalk ratio of less than -20 dB. It is shown that this device could be applied to switcher and optical sensor.