A new approach for the measurement of radiation patterns for passive RFID tag antenna under chip-loaded condition using Friis equation is proposed in this paper. Different from the conventional situation where the antenna is matched to 50ω, the RFID tag antennas are usually designed to match to the conjugate chip impedance for maximum power transfer. Besides, the impedance of the tag IC usually varies with received power and thus traditional way of characterizing the radiation patterns and gain of the tag antenna using 50ω system may not be adequate. Based on the Friis equation, the proposed approach provides a straightforward way of determining the radiation patterns of tag antenna under chip loaded condition. The proposed approach has been applied to the measurement of two types of commercially available tags and proven effective.