The influence of a constant temperature and humidity treatment, as well as soaking temperature on thickness swelling （TS） rates for wood-based panels, was investigated in this paper. The results showed that, for medium density fiberboard （MDF） and particleboard, their thickness swelling rates increased when the soaking temperature was increased, and there was a linear relationship between them, whereas, plywood was influenced little by the soaking temperature. Also studied was whether treating the specimens in a constant temperature and humidity condition before soaking would influence TS test results. It was also found to be essential to give a conditioning treatment for particleboard and thin MDF to ensure correct test results.