In case of evaluating stability of soft ground using circular slip analysis, it is very important to determine the undrained shear strength (S u) of the ground. Meanwhile, fibrous and high organic peat is distributed widely in Hokkaido, the northernmost island of Japan. As peat is accumulated heterogeneously, unconfined compression tests, vane shear tests and other tests performed for only a few samples lack validity. The electric Cone Penetration Test (CPT) is more reasonable and valid than these, in that CPT can estimate average S u from in-situ tests, which continuously provide information. A series of K 0 consolidated-undrained triaxial compression tests (K 0CUC tests) on undisturbed peat soil collected at several sites in Hokkaido was conducted. Also the CPT was performed at the same sites. This report describes the relationship between S u obtained from the K 0CUC tests and CPT cone resistance (q t), and proposes a method for estimating S u in peat soil from q t.