The adaptive optics systems are needed to deal with the effects of aberration produced by different reasons. Thus, such a system calls for fast wavefront sensing techniques. In the present report a technique based on an interferometric principle using a cross slit and a lenslet array is proposed. A CCD camera captures the resultant fringe pattern in the back focal plane of the lenslet array and further analysis may be made using computer software developed. The advantage of this method lies in the improved accuracy as compared to normal Hartmann sensor due to the interferometric fringe analysis.