In order to evaluate the applicability of an optical-fiber sensor system in the instrumentation of piles, laboratory and field tests were conducted. A multiplexed sensor system, constructed by arranging several Fiber-Bragg-Grating (FBG) sensors along a single optical fiber line, is capable of measuring local axial strains as a function of wavelength shifts. From the strains measured by FBG sensors, the distributions of axial load in three model piles and a field test pile are found to be comparable, in terms of both magnitude and trend, to those obtained from conventional strain gauges. The result of an application suggests that the FBG sensor system is an effective tool for the analysis of the axial load transfer in piles. Copyright ASCE 2006.