Atomic order thermal nitridation of Si1-xGex(100) (x=0, 0.4, 1) by NH3 gas environment at low temperature of 400 °C has been investigated using an ultraclean low-pressure chemical vapor deposition system. After NH3 exposure (NH3 partial pressure of 550 Pa for 30 min), it is found that Si(l00), Si 0.6Ge0.4(100) and Ge(100) are nitrided in atomic-order. In the case of Ar purging at cooling period after the nitridation, N atom amounts on Si(100) and Ge(100) are 8.0×1014 and 5.5×l0 14 cm-2, and the amount on Si0.6Ge 0.4(100) is 12.5×10114 cm-2. In the case of H2 purging at the cooling period, it is found that slight reduction of the N atom amount on Si(100) down to 5.6×l014 cm-2 (70 %) is observed, although the N atom amount on Ge(100) is significantly reduced to 1.2×1014 cm-2 (22 %). On the nitrided Si0.6Ge0.4(100), it is also found that such N reduction phenomenon by H2 (down to 30 %) is observed only at the site of the nitrided Ge atom, although the N atom amount is much smaller at the site of the nitrided Si atom. copyright The Electrochemical Society.