The conservation of biodiversity in Mediterranean Chile requires a greater coverage than that represented in the Protected Wild Areas of the State (SNASPE) and Private Protected Areas (APP) are complementary alternative for conservation. The study of biodiversity in APP is scarce, ignoring its contribution to the conservation. The small mammals assemblage of the APP Bioparque Puquen and surroundings was described comparing the species richness, relative abundance and diversity index of Margalef inside and outside the park. Using the Morisita-Horn similarity index and a cluster analysis, the small mammals assemblage was compared between sampling points categorized in different biotopes. Endemic and native species were more abundant outside the APP. There were no significant differences in specific richness or diversity inside and outside the APP. The small mammals assemblage was grouped into biotopes of dense native vegetation and prairies, ravines and seasonal waters. The park would act as buffer area, protecting the nearby areas of the Los Molles-Pichidangui Priority Site. The importance of rocky outcrops as a habitat for endemic species of small mammals and protecting the areas adjacent to the BioParque is discussed.