Localization schemes based on RFID technology are mostly estimating the location of tags either in a two dimensional or in a three dimensional space. In some applications, however, only the relative positions of the tags are needed instead of the exact locations. In this paper, we propose a RFID based localization scheme that estimates the row and column indexes of tags that are spread in a two-dimensional matrix using the movement of an RFID reader. The movement of a reader brings changes in the set of tags that are scanned in consecutive read operations. Based on the changes, the proposed scheme estimates relative positions of tags in a tag matrix. Proposed scheme does not need additional equipment such as reference tags and readers. Therefore it can be easily applied to various applications that need relative positions among tagged objects. Localization of passive RFID tags in a two dimensional space.Estimation of relative tag positions using a moving RFID reader.The proposed scheme can be useful for practical usages.