Unimolecular dissociation rates of energized molecules influenced by the slow intramolecular vibrational energy redistribution (IVR) an formulated for no-barrier potentials of the reaction coordinate R. The master equation as to states projected on the reactive mode is presented and is solved by reducing the equation to an equivalent diffusion equation. An approximate solution for the steady state condition gives the generalized dissociation rate constant k(D) which is expressed as k(D)(-1) = k(diff)(-1) + k(de)(-1) + k(RRKM)(-1), where k(diff), k(de), and k(RRKM) represent the internal energy diffusion rate constant, equilibrium barrier crossing rate constant by bound-continuum transitions, and the fragmentation rate constant corresponding to the flux which crosses the critical configuration R = R-double dagger, respectively. The former two rates, which are due to IVR, are expressed by the transition kernel between states of the reactive mode, and the latter gives the RRKM rate which is valid in the rapid IVR limit. The rate limiting steps for various reactions are discussed.