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Fe57 Mössbauer spectra were taken from 300 K down to 4.2 K for two single-crystal samples of Y1.8Er0.2Fe14B, one cut with its  axis normal to the surface, the other with its diagonal of , , and  axes normal to the surface. The former sample has shown that the angle between the Fe spin and the  increases continuously below ~90 K. This qualitatively agrees with the spin reorientation (i.e., the easy axis of magnetization rotates away from the  toward  on cooling below ~90 K) which have already been observed by torque magnetometry. The latter sample has shown that the Fe spin actually rotates continuously; abrupt change of Fe spin direction, which have been already suggested to occur in Er2Fe14B, does not occur in Y1.8Er0.2Fe14B. The samples were so thick that the intensity of strong absorption lines were saturated. We have derived a formula with which the saturation effect can be corrected in order to determine the angle of the Fe spin direction correctly.