The cumulative reaction probabilities are calculated for the dissociation and isomerization reactions of formaldehyde in the ground state by the use of the Watson Hamiltonian. We devise an efficient computational procedure for treating the vibrational angular momentum operator. We employ an improved global potential function at the CCSD(T) level derived from the MP2 level one. The resultant cumulative reaction probability for the dissociation channel is similar to1.5 times larger than that for the isomerization one at the energy region of interest. It is also found that the transition state theory reproduces the result with the full Watson Hamiltonian within 10% error. (C) 2004 Elsevier B.V. All rights reserved.