Abstract: The strain gradient field in the neighborhood of a Mode-I crack in an infinite plate under uniaxial tension is formulated based on the solution put forward by Irwin in the linear elastic fracture mechanics, and the relation between the strain gradient and the crack extension is established. Flexoelectric relationship between the electric polarization and the strain gradient is also presented, and a method of Mode-I crack monitoring to obtain the coordinates of the crack tip and the length of the crack extension using strain gradient sensors is proposed. This study provides some necessary theoretical preparations for implementing the strain gradient sensors, by which the crack extension in engineering structures can be monitored in a real-time manner. Flexoelectricity sensing technology has broad prospects in the field of structural health monitoring.