We have analyzed decay kinetics of CF2 radicals in the afterglow of low-pressure, high-density C4F8 plasmas. The decay curve of` CF2 density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10(-4). This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF2 radicals (CF2+CF2-->C2F4). The rate coefficient for this reaction has been evaluated as (2.6-5.3) x 10(-4) cm(3)/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.