We have proposed an optical probe that can be used to characterize the orientation of collagen fibers in human dermis. A specific probing ability for collagen results from the use of second-harmonic-generation (SHG) light induced by collagen molecules in the tissue. Based on the concept of collagen SHG light, a reflection-type polarization measurement system (named SHG polarimetry) with a probe light spot of 15 μm in diameter has been constructed, and the human reticular dermis has been measured using this system. Resultant data exhibit that the reticular dermis possesses approximately uniaxial orientation of the collagen fibers. Furthermore, we demonstrated a nondestructive measurement of the collagen orientation in the papillary dermis across an epidermis layer. For distribution measurement of the collagen fiber orientation in the reticular dermis, we have extended the SHG polarimetry to one- (1-D) and two-dimensional (2-D) measurement. By the macroscopic 2-D SHG polarimetry, we have observed that the orientation angle and organization degree of collagen fibers vary widely depending on the discrete probing positions in the reticular dermis. Furthermore, microscopic 1-D SHG polarimetry indicated a swell of the orientation angle and a large variance of the organization degree in the collagen fibers in the microscopic region. These results imply that the reticular dermis posses a tangled structure of collagen fibers, which is highly consistent with the result of the anatomical examination of the skin. The proposed method will be a powerful tool for monitoring the microscopic distribution of the collagen fiber orientation in the human dermis. © 2004 Society of Photo-Optical Instrumentation Engineers.