We prepared RuO2 thin films on Si substrates by pulsed-laser deposition using a sintered oxide target, and investigated the crystallographic nature and the thermodynamic stability of the films in terms of the growth conditions: temperature and oxygen pressure. Faceted grains grew in a highly oxidizing atmosphere. The films grown at room temperature included a large quantity of oxygen ions in the Ru metal matrix. An oriented Ru metal film grew under a non-oxidizing atmosphere. The grain morphology was closely related to the oxygen pressure.