The thermodynamic stability of clathrate hydrates encaging nonspherical ethane and ethylene molecules as well as spherical Xe atoms has been investigated by grandcanonical Monte Carlo (GCMC) simulation. In addition to most of the static properties obtained by a standard Monte Carlo simulation, cage occupation ratios at various pressures are calculated by GCMC simulation where guest molecules are allowed to be created or deleted. Thus, we can evaluate the free energy of cage occupation at given guest pressure. This indirect method provides an alternative way to predict the stability of a variety of clathrate hydrates with simple GCMC simulation runs without invoking normal-mode analysis and thermodynamic integration when the chemical potential difference of water between ice and empty hydrate is known.