The cut-back method is the standard test method to check the attenuation of optical fibers. The advantages of this method are low uncertainty, good reproducibility, and an applicability in a broad spectral range. But for some applications, especially in the field service, the optical time domain reflectometry seems to be more useful because (1) this method is non-destructive, (2) measurements can be made from one end of the fiber, and (3) the back-scattered signal contains information about the longitudinal homogeneity of the fiber or the fiber system. For an approval of this technique as a second standard test method, an uncertainty of 0.01 dB/km of the attenuation coefficient measurement is required. This small uncertainty demands a calibration of the loss scale of the optical time domain reflectometer (OTDR) used. Therefore, a calibration procedure is proposed using a standard fiber as a scale unit. The specification of this fiber, the preparation as a standard and its calibration in an accredited calibration laboratory, are discussed. An uncertainty of about 0.005 dB should be achievable in attenuation measurement of the standard. The calibration of the power scale of the OTDR with the aid of transfer standard, lead-in fiber and/or attenuator, and a proposal for linearizing the scale of power response are presented.