We propose an RF tone in-band labeling technique that is able to support large-scale and low-latency optical packet switch. This approach is based on N in-band wavelengths, each carrying M radio frequency (RF) tones. The wavelengths and the tones have a binary value, and are able to encode 2N×M possible routing address. We develop an optical label processor for the RF tone in-band optical label based on parallel and asynchronous processing. It allows the optical packet switch with an exponential increase of number of ports at the expense of limited increase in the latency and the complexity. By using RF tone in-band labeling technique, we demonstrate error free (bit error rate <; 10-9) optical packet switching operation for both 160 Gb/s packets and 40 Gb/s packets. We further investigate the scalability, the latency and the optical power fluctuation tolerance of the proposed RF tone in-band labeling technique. We show that 30 label bits are able to be delivered using single in-band wavelength, and the label processor introduces an extra latency of less than 7 ns.