The deeper a reservoir is, the smaller diameter the drilled full diameter cores have. It is difficult to conduct insitu stress experiments with conventional methods if the diameter of the full diameter cores is less than 6.0 cm, especially for cores abundant in natural fractures. In this paper, based on conventional Differential Strain Analysis (DSA) methods and wave velocity anisotropy methods, we developed an improved Differential Strain Analysis (DSA) method that is specially designed for small full diameter cores. The improved method, combined with the paleomagnetic core reorientation tests, can predict magnitude and orientation of in-situ stress. Results from the improved method are very close to those obtained from conventional Differential Strain Analysis. The improved method was applied to carbonate cores in Tahe Oilfield, which has a depth up to 6000 meters and has cores with a diameter from 5.5cm to 6.0 cm. The experimental results with the improved method show good consistence with the field-monitored ones, which shows that the improved method is reliable and practical.