Isothermal changes in effective and pore pressures of a normally consolidated unsaturated clay during perfect sampling were analyzed theoretically. The residual effective stress and the degree of saturation of an unsaturated asample are, in general,smaller than those of a saturated one, and affected by the amount of contained air and the expansibility of the soil skeleton. In extreme cases the residual effective stress is possible to be negative. The residual effective stress of an unsaturated sample after storage is always positive and usually very small. For the UU specimen of a perfect sample the residual effective stress is equal to that of a perfectly saturated perfect sample when the pore pressure of the specimen is identical with that of the in-situ sample. The final residual effective stress of a stored UU specimen at a higher confining pressure is same as that of the UU specimen immediately after sampling. Finally, the above theory was applied to the analysis of the field test results.