Rock surface strength in relation to surface biological covers was examined on outer walls of the first gallery in Angkor Wat temple. The surface covering condition on the outer walls was categorized into three areas: 1) covered area; 2) uncovered area; 3) exfoliated area. Both initial and mean rebound values of Schmidt hammer on covered area were lower than those on uncovered and exfoliated areas. SEM observation of the rock fragment showed that the black cover on the outer walls is biofilms composed of filamentous materials. Cracks extending beneath some black biofilms are the cause of exfoliation. These results indicate that weathering processes occur beneath the biofilms, and bioprotection is not likely to work on the outer walls of the first gallery in the temple.