Loss of radon-222 in water sample stored in a polyethylene bottle was analyzed. The radon content was measured by a liquid scintillation spectrometer. The radon content (corrected for radioactive decay) was plotted against the storage period in the polyethylene bottle, and the decrease with the lapse of time was observed. The loss of radon was caused by adsorption on polyethylene surface. The radon content decreased according to the first order kinetics. Apparent rate constant of the adsorption was proportional to the surface area of the polyethylene and to the reciprocal of the sample water volume. The proportionality constant was obtained as a function of temperature, K=10200 exp (-4382/T) em·h^-1. Apparent activation energy was 36 kJ·K^-1·mol^-1. The result supports physical adsorption. On the basis of the result, correction for radon content of a sample water stored in a polyethylene bottle was made possible.