Objective To develop an improved DIC (digital image correlation) algorithm suitable for measuring large ROM (range of motion) of the cervical spine, as traditional DIC algorithm is not capable of accurately measuring ROM of the cervical spine due to its large rotation angles. Methods An algorithm which allowed rotation of the subset window was proposed to achieve robust correlation matching in the measurement. A new iterative variable, which represented the orientation of the subset, was introduced and incorporated in the Newton-Raphson iteration method together with the position variables (x,y). By assigning an initial guess to these variables individually, the precise location and rotation angle could be determined in the deformed image. The precision of the proposed method was evaluated by translation and rotation experiments. ResultsThe translation experiment confirmed that the proposed method had the same accuracy as the traditional DIC, and the displacement measurement accuracy was within 0.5%. While the rotation experiment indicated that the proposed method could measure the deformation at any angles with precision less than 0.5°. The method was then applied to the measurement of ROM of cervical spine subjected to compressive loads and received good results. Conclusions Compared with the traditional DIC algorithm, the proposed method can achieve accurate measurement with large ROM for cervical spine tests with different loads, and provide an effective means for assessing the stability and physiological activities of cervical spine.