Surface morphologies of Pb films prepared on Si(111)-7 × 7 substrates are investigated by low temperature scanning tunnelling microscopy. Depending on substrate temperature, coverage and post-deposition annealing temperature and duration, three kinds of morphologies, uniform films, interconnected islands and isolated islands, are formed. In all three cases, preferred heights/thicknesses are observed due to the quantum size effect (QSE) along the surface normal direction. The formation and lateral distribution of either isolated islands or interconnected islands are found to be driven by competition between the boundary formation energy and the long range dipolar repulsive interaction between boundaries. With increasing substrate temperature, the critical thickness for stable films increases, which we attribute to the competition between the long range force induced by the QSE and the elastic force associated with thermal annealing.