Privacy has been the key road block to cloud com-putting as clouds may not be fully trusted. This paper is concerned with the problem of privacy-preserving range query processing on clouds. Prior schemes are weak in privacy protection as they cannot achieve index in distinguishability, and therefore allow the Cloud to statistically estimate the values of data and queries using Domain knowledge and history query results. In this paper, we propose the first range query processing scheme that achieves index in distinguishability under the in distinguishability against chosen keyword attack (IND-CKA).Our key idea is to Organize indexing elements in a complete binary tree called PBtree, which satisfies structure in distinguishability (i.e., two sets of data items have the same PBtree structure if and only if the two sets have the same number of data items) and node in distinguishability (i.e., the values of PBtree nodes are completely random and have no statistical meaning). We prove that our scheme is secure under the widely adopted IND-CKA security model.