Pre-purchase and post-purchase item information provision methods and systems using an RFID technology and computer readable storage media storing programs for executing the methods are provided. A pre-purchase item information provision method for a radio frequency identification system according to the present invention includes transmitting, if a mobile reader is detected by a local server installed in a salesroom, a certificate containing information on a location of an object information service server and an authentication value to the mobile reader; transmitting, at the mobile reader, a query requesting an item ID to a tag attached to a target item; transmitting, at the tag, a item ID information generated by encrypting the item ID using the authentication value to the mobile reader; transmitting, at the mobile reader, the item ID information and the authentication value to the object information service server; and transmitting, at the object information service server, data retrieved in a database in correspondence to the item ID, the data being retrieved when the authentication value exists in the database. The item information provision method and system of the present invention can guarantee reliability of information on an item in the pre-purchase state and prevent the information from being eavesdropped in both the pre-purchase and post-purchase states. Also, the item information provision method and system can prevent the tag ID from being tracked, thereby protecting purchaser's privacy, preventing the tag from being forged, and improving computation efficiency.