The absorption spectrum of the vibrational excited state in BC13 has been obtained by means of a infrared double resonance technique. A variety of relaxation processes and energy transfer processes were observed. The relationship PτV-V(11BCl3)= 3μs·torr has been obtained. A radial sound modulation was observed on the double resonance signal. The sound velocity is 2×104cm/sec and it agrees with the calculated value.