Abstract: The characteristics of rockmass block are the main indexes, which are usually represented by RQD. But RQD has some disadvantages. The probability distribution of basic rockmass block, block percentage, RQD t, and rockmass block index (RBI), as the important supplements to defining the rockmass block, are also used to describe the characteristics of rockmass block, which are representative of the rockmass integrality more allsidedly and effectively and enrich the assessment system of rockmass quality. Based on the investigation and statistics of the rockmass block at dam foundation of Xiluodu Water Power Plant Project in the downstream of Jinshajiang River , the rockmass block characteristics, such as the probability distribution of basic rockmass block, block percentage, RQD t and rockmass block index (RBI), were deeply studied.