The optical time domain reflectometer (OTDR) displays time-dependent optical power when light is reflected back to the input edge of an optical fiber, and it can show an abnormal point of an optical fiber and also measure an optical loss. Hence, OTDR has become a useful tool at the time of optical fiber fabrications and also maintenance of optical fibers. Thus, considerable work has been done on the high performance such as dynamic range and high spatial resolution of OTDR. This paper discusses the enhancement of OTDR using optical fiber amplifiers. First, on the basis of signal-to-noise ratio (SNR) calculation, it is demonstrated that the application of an optical amplification as a post-amplifier for OTDR is quite useful. Next, a two-stage cascade series Er-doped fiber amplifier is realized. By cutting off the spontaneously emitted light by means of a fast switching, the output light of 30 dBm is obtained. Finally, in the case of a direct detection OTDR using the optical fiber amplifier, 30.5 dB dynamic range is obtained for 1-μs pulsewidth and 216 integration. For the coherent detection OTDR, 36 dB is obtained for 1-μs pulsewidth and 212 integration. Based on these results, the application of optical fiber amplifiers to OTDR is demonstrated clearly to be powerful.