In the west part of Tokyo, Japan, giant flying squirrels (Petaurista leucogenys) eat the leaves of Quercus trees. In this area, there are 12 species of Quercus trees, including 5 deciduous and 7 evergreen species. We conducted weekly censuses of a 2 km fixed route, collecting leaf debris with feeding marks by the flying squirrels, for three years. Food availability was also estimated as the total cross-section area at the breast height of each tree species. Giant flying squirrels most preferred the leaves of Q. acutissima (one of the deciduous trees) during spring to summer, while they most preferred the leaves of Q. sessilifolia during autumn to winter when the deciduous trees were all unavailable. To examine reasons for their leaf preference, sugar and total phenolics were measured for leaves of all 12 species of Quercus. Sugar contents were highest in the leaves of Q. acutissima, suggesting that the flying squirrels prefer sweeter leaves. The total phenolic contents of leaves were positively correlated with the sugar contents among 12 Quercus species, so that the leaves of Q. acutissima included the highest total phenolics. However, the flying squirrels did not avoid eating leaves of this species. On the other hand, in autumn to winter, evergreen Quercus species did not include such high sugar and total phenolic contents, suggesting that factors other than sugar and total phenolics influence their tree leaf preference among the evergreen Quercus species.