This paper introduces PDT (Pulse Discharge Technology) piling method. The PDT piling method is to install in-situ piles using electrical power so called Pulse power. Since this piling technique can make not only a bore hole to be expanded but also the ground to be improved by compaction, the pile installed by PDT appears to be able to develop shaft and end bearing capacity efficiently. In this study, we introduce a coefficient of expansion. In order for quantitative evaluation of the relation between pulse power and ground expansion a series of in-situ tests are conducted. In the tests, we divide soil conditions into several groups by constituents and the number of blows in standard penetration test, N. The tests are carried out on various numbers of pulse discharge. As a result, it is concluded that the coefficient of expansion appeared to increase with an increase of the number of discharge and with a decrease of the number of blows, N. In addition, the coefficient of expansion appears to differ from one position to another even though having the same N value.