This paper reports the design, implementation and the result of real-world experiment of wireless modal testing with battery-free backscatter sensors. The battery-free backscatter sensor comprises a custom radio frequency integrated circuit (RFIC), a commodity micro electro-mechanical system (MEMS) accelerometer, and an antenna. Sensor data concurrency demanded in modal analysis is realized by allocating a dedicated subcarrier to each backscatter sensor. The orchestration of wireless communications in 920 MHz based on Gen2 protocol and the wireless power supply to the backscatter sensor are performed by a software defined interrogator built on Lab-VIEW communications and USRP. We developed an over the air (OTA) subcarrier frequency allocation with a sequence of WRITE commands of Gen2 protocol, and its performance was confirmed with the experiments in an indoor radio propagation. The accuracy of the proposed wireless structural modal testing was experimentally verified by comparing with a commercial modal test system with wired accelerometers. Although the phase characteristic measurement is disturbed by the instability of the clock source of RFIC, the gain characteristics of wireless modal testing matches well with that of a commercial wired modal testing system.