Abstract：Kaiser effect is widely used in geostress determination in petroleum industries，while other typical methods are restricted for great depth and complex wellbore conditions. During the measurements of geostresses at great depth with Kaiser effect，sampling deviation possibly has some effects on geostress due to irregular shapes of full-scale core and errors in preparing samples，which may be ignored by users and is not considered yet. Supposing that the sampling orientations of 0°and 90° are both accurate and there is some small deviation in the orientation of 45°，a simplified mechanical model is built up based on elasticity theory and series expansion. A series of experiments were conducted with homogeneous reservoir sandstone by changing the sampling deviations；and the comparisons between experimental results and mini-frac interpretations indicate that Kaiser sampling deviations have effects on geostress in quadric relations，which agrees well with the proposed model. When the sampling deviation angle is less than 5°，the effect can be neglected，but great deviation angles have significant influences that can not be neglected；and the changes of geostress are about 2.5 MPa when sampling deviation is 15°. The effect changes with different referenced angles in non-directional full-scale cores. As for irregular cores at great depth，it will be caused by mechanical damage and natural fractures；and it is impossible to get perfect three samples strictly in direction of 0°，45°and 90°，respectively. Thus the proposed model has its priority and can be used to evaluate the measurement errors of geostress.
