Based upon cause analysis of the low-level jet along the east side ofthe Rocky Mountains the concept of 《Interface Effect》 is established. Ithas been found that the basic reason for the formation of the low-leveljet in North America is the interface effect, whose mechanism is pushingby geostrophic wind, and thereby air columns in the lower troposphereare compressed between two surfaces-the ground and the bottom ofthe inversion-which slope with different patterns in a vertical cross-section normal to the jet stream. As a result, the air parcel is acceler-ated along the streamline and the anticyclonic shear of the current in-creased. Also, the diurnal variation of the jet is determined by the in-terface effect.