The effect of the free volume on damping properties for CIIR (Chlorinated isoprene-isobutylene rubber) and CIIR/PEA (polyethylacrylate) were investigated by positron annihilation lifetime spectroscopy and DMA, respectively. The DMA results indicate that the tanδ peaks of CIIR and CIIR/PEA both have very broad covering temperature range. The tanδ peak of CIIR/PEA shifts to higher temperature region which indicates that the CIIR/PEA has better damping property in high temperature region that CIIR. From PALS results, we found that the lifetime of o-Ps in CIIR and CIIR/PEA both exhibit broad glass transition. It is interestingly that the glass transition region of CIIR/PEA also shifts to higher temperature than CIIR. This character of the free-volume is well in agreement with the damping properties obtained by DMA. It indicates that the free volume has close relationship with damping mechanism for CIIR and CIIR/PEA.