Natural disasters affect thousands of people every year. In a large disaster area, search and rescue operations can face great difficulties to locate victims. In this paper, we propose a system, called Krypto, with UAV to assist search and recue operations. By flying over disaster area and detecting wireless signals from any cellular phones, Krypto is able to locate possible victims. In addition, this work addresses the challenges of maximizing the searching area and minimizing the location errors with different searching paths. We have analyzed different searching paths in terms of coverage, location errors, average speed, average searching time, and power consumption. Our experiments presented the design considerations and the performance comparisons of different searching paths for finding victim in a large disaster area.