A general method of obtaining hollow nanoparticles by utilizing the Kirkendall effect has been reported recently. We examine here the thermal instability of hollow nanoparticles. The difference of vacancy concentrations at the inner and outer surfaces of a nanoshell will generate an outflux of vacancies and transform it into a solid nanoparticle. Phenomenological as well as kinetic Monte Carlo modeling has been applied to analyse the shrinking kinetics of nanoshells, consisting of either a pure element or an intermetallic compound with large difference of partial diffusivities of the components. Shrinking kinetics and time to collapse to solid particles are demonstrated to be determined mainly by the slow diffusant, making compound nanoshells more stable than anticipated.