Collagen-sensitive SHG microscope equipped with an 800-nm mode-locked Ti:Sapphire laser has been employed for in vivo visualizing of the dermal collagen fibers in mouse skin16 and human skin.17 Furthermore, European conformity-certified multiphoton microscope, giving two photon fluorescence (TPF) images of NAD(P)H, flavins, porphyrins, elastin, and melanin as well as SHG image of collagen, is commercially available for clinical instrument.18 With respect to assessment of dermal photoaging, combined SHG and TPF microscope gives an indicator of changes in dermal collagen and elastin content ex vivo19 and in vivo20 because degradation of collagen and excessive deposition of abnormal elastin are observed in photoaged skin. Very recently, the elastin-to-collagen ratio has been introduced as a new indicator for human dermal skin aging.21 On the other hand, a mode-locked Cr:F laser centered around 1250 nm has emerged as a new source for SHG microscope, showing decreased photodamage22–24 and enhanced penetration depth.25 The Cr:F laser-based SHG microscope has been successfully employed for imaging of dermal collagen fiber in excised mouse skin,26 excised fixed human skin,27 and in vivo human skin.25,28 From the viewpoint of assessment of skin aging, since Cr:F laser-based SHG microscope enables us to probe down to deeper portions in the dermis-that is, the reticular dermis-it may contribute to a better understanding of age-related structural change in the dermal collagen fibers. For example, based on the fact that the efficiency of SHG light is sensitive to the collagen orientation when the incident light is linearly polarized, polarization-resolved, Cr:F laser-based SHG microscope has been applied to investigate the relation between wrinkle direction and collagen orientation in excised UV-exposed mouse skin.29 However, few reports have revealed that aging-related structural changes in human dermal collagen fibers are clearly visualized in vivo in the form of high-contrast SHG images. In this article, Cr:F laser-based SHG microscope was used for in vivo visualization of aging-specific structural changes in human facial collagen. Furthermore, quantitative image analysis using Fourier transformation of SHG images was performed to identify characteristic structures in dermal collagen fibers with and without skin aging.