The dissociation rate constant of the water dimer was calculated by using the method proposed by Yao and Lin (YL method). The dividing surface method and RRKM theory are used to calculate the pseudo transition state and the rate constant, respectively. For the canonical case at temperature range of 243-1000 K and for the microcanonical system at total energy 1411-4000 cm(1), the anharmonic rate constant, around 10(9)-10(11) s(1), is close to the experimental prediction. The present results indicate that the anharmonic effect should be included and YL method is suitable for calculating dissociation rate constants of such small flexible water cluster. (C) 2009 Elsevier B. V. All rights reserved.