The distance at which animals start to flee from approaching threats should reflects the degree of fearfulness, and thus, provides a useful measurement to evaluate animal personality and tolerance to human disturbance. Such metrics measurements, however, are mostly limited to open, high visible habitats, such as grasslands and urban parks. Alternative measurements are required for other types of habitats, such as typical forests. For arboreal species, we expect that a vertical escape distance (VED), the height at which animals stop climbing in a tree toward approaching threats, would reflect animal fearfulness. We compared VED and two commonly used metrics, alert distance (AD) and flight initiation distance (FID) in the Eurasian red squirrel (Sciurus vulgaris) towards human approach. We found that VED was significantly related with FID, but not AD. Data collection rate in VED was two to three times higher than that in the two previous metrics in vegetated areas. In natural environments, VED would also reflect the degree of fearfulness in arboreal species.