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We demonstrate an ultrahigh-resolution optical time domain reflectometry (OTDR) system by using a mode-locked laser as the pulse source and a linear optical sampling technique to detect the reflected signals. Taking advantage of the ultrashort input pulse, the large detection-bandwidth, as well as the low timing jitter of linear optical sampling system, a sub-mm spatial resolution is achieved. As the pulse-width is broadened with the increase of distance due to chromatic dispersion and large bandwidth of the ultrashort pulse, by adopting digital chromatic dispersion compensation, we achieved a spatial resolution of 340 μm when measuring the reflector at 10 km. This technique helps OTDR find new foreground in long-range and ultrahigh-resolution distributed applications such as remote optical identification device detection for diagnosing passive optical network links, or precisely detecting fault positions in aircrafts.